As I suspected, there was a little more to the story than the abstract (below) indicated.  There's almost a disconnect between the conclusion in the abstract and the actual contents of the paper.

First off, the breed identifications were casual - based on owner attributions upon licensing, not on actual verification of purebred status.

Dog bite studies, including that of this report, have inherent limitations. Besides the known underreporting of dog bites, breed-specific population estimates are lacking.14,30 Dog license data can only provide an estimate of breed-specific population denominators. In 1987, 43,650 dogs were licensed in Multnomah County, but a countywide survey placed the true number of owned dogs at 101,794.31 Breed assessment is often subjective21,29; license forms and bite reports rely on owners and injured persons to describe the breed. Bites from large-breed dogs, especially pit bull–type dogs, Rottweilers, and German Shepherd Dogs, are more likely to be reported and result in medical care, which would overrepresent those breeds among biting dogs—in other words, creating reporting bias.14-17

Therefore, the purebred finding is of little significance and as usual with these kinds of studies, I don't know why it was attempted - although in this case, the authors wisely decided against actual breed attributions and stuck to groups - working, herding, etc.  Still, there is no evidence of actual purebred status among the dogs studied.  Here are the groups, I forgot to include them earlier:

As for the statement that intact male dogs were responsible for the majority of bites, that too is open for discussion.  Here's why:

                                       No. of Dogs              No.Bites         Bite rate/1000

                                        in Cohort                                          licensed             Risk Ratio

Sex

Spayed female                  21,584                          64                      3.0               Referent

Sexually intact male          3,157                         174                    55.1               18.6 (13.9–24.7)

Sexually intact female       2,028                           63                    31.1               10.5 (7.4–14.8)

Neutered male                 19,991                         153                      7.7               2.6   (1.9–3.5)

Unknown†                        1,090                         182

They are dealing with licensed dogs only and in the paper they estimate that there are just under 102,000 dogs in the area, which is probably low.  They have licensed about 39% of dogs in the area (42,526).

However, I note that the 'unknown' cohort, with roughly 1/3 the number of dogs as the intact male cohort, has more actual bites.  Therefore, it looks as though unknown is the problem cohort.

Based on the available study cohort (group) of each category, yes, intact males appear to lead based on number of dogs and number of bites but it is really all in how you interpret and catgorize the findings.

I think the likelihood that an intact dog could be owned by someone who doesn't seek routine veterinary care is valid, since sterilization is always brought up at clinics these days.  Not seeking veterinary care is a sign of being less responsible than the average owner.  Sterilizing a dog will not change that.

Unknown does not show a bite rate or calculated risk ratio, which is unfortunate, but bite rate/1000 would be a whopping 170 or so.

Overall though, the dog bite problem seems well controlled in the area.

Regarding the low income finding, here's a snip which confirms my conclusions and also supports KC Dog Blog's excellent findings:

Dogs living in a lower income neighborhood may not be afforded the necessary training or supervision needed to minimize a high bite-risk situation. In another study29 examining dog bite injuries in St Louis, Mo, bite injuries occurring in low income areas were attributed to large numbers of children playing outdoors, few homes with adequate fencing, poor dog control, and a high proportion of large-breed dogs owned for protective purposes,

In view of the above, I fail to see how the conclusion in the abstract was reached, ie, that intact males, purebreds and those owned by low income families were the dogs that should be targeted by officials.

I also note that the high sterilization compliance in the area described by the numbers provided would obviate any sort of further impetus or legislative effort to increase the number of compliant owners.

In closing, the authors quite reasonably state:

The human and veterinary medical communities and animal control agencies need to work together to help foster healthy relationships between people and their pets, especially in low income neighborhoods.21,26,29 Pediatricians in particular can play a vital role in injury-prevention counseling for children and parents (in both dog-owning and non–dog-owning families) during routine medical visits.32 Lowcost spay and neuter surgeries need to be easily accessible in the community, and education efforts should target dog owners through pet stores, groomers, boarding facilities, and other stakeholders such as postal workers and animal control agencies. Animal control agencies should be supported to maximize regulatory and educational activities.  Nevertheless, owners are inevitably liable for the actions of their dogs and need to make every effort to minimize their dogs’ bite potential through obedience training; neutering; and supervision, especially around children