"I weighed the advantages against the disadvantages," Peter replies, "and I found helping you more to my advantage." It's a non-answer, but really the only kind an Observer would probably respect. Cold, logical, and completely beholden to that logic, desires be damned.

He doesn't need to know the whole truth, that Peter knows rounding up the rest of the Observers is the first step to eliminating them. After he utilizes their knowledge, of course, to find a way out of this hellhole.