It's the difference between theoretical risk and actual risk, a distinction that many people chose not to see. The more partners you have, and the more partners your partner has, the more vectors there are for disease, theoretically. However, in my experience, poly people tend to be very responsible about the issue of sexual health, including regular testing, honest conversations about past sexual histories and safer sex methods, more so than most of the mono people I know. I'm sure there are very responsible mono people and not so responsible poly ones, but overall, I think poly people tend to be more responsible, because more people are at risk of there is a problem.
In short, is a poly person at more risk of contracting an STI and a monogamus couple that only ever had sex with each other? Yes. Is the average poly person have more actual risk than the average mono person? Probably not.
I do hope someone comes in with actual data, as that would be really interesting.