I can see this playing out...

--Guest books at hotels.com for $100 including internet and breakfast.

--Guest then calls Marriott and invokes BRG; Marriott says "we can do $75 without internet and breakfast." Guest says cool - I'll take it.

--Guest then checks in at Marriott and says, "where's my free internet and breakfast?!? You guys are unethical!!!!!"

I think scenarios like this are why Marriott and every other hotel brand would not directly price-match unless comparing exact apples-to-apples bookings.