This isn't quite how it works. It's entirely possible to get PC points from a reservation booked at a TPI site. Each rate code that the hotel has will have a field for whether or not PC points are paid out on that rate code or not. Now the reason that you don't often see points from a TPI booked reservation is because all of the TPI specific rates have a no in the PC points field.

There still are some rate codes that TPI sites have access to that do grant points but from what i've seen, a TPI site will post up the cheapest rate they have access to. The idea there being to remain competitive with other TPI sites.

Another reason people might not get points is that their PC # isn't on the reservation. I haven't booked often through TPIs but from what i've seen, there isn't a place to enter your PC# so you have to have the number added to the res after booking it.

So the rate has to allow points, which is tough but does happen and your number has to be on the res.