Despite my forum title, I don't work for TGC. I do have a decent understanding of business though...
Firstly, I would qualify this by saying it's free for anyone prepared to sign up to an additional service. It's been happening since the 1920's when companies started giving incentives for buying their product.
Think of it the other way around (which is what it
actually is) - FPSC is the free toy in your cereal packet, or the free CD in your newspaper - you just have to buy the cereal / newspaper first.
Secondly, anyone who has already bought FPSC did so because it was a worthwhile investment for them at that time. You could have got it free/cheaper today, but you wouldn't have been able to use it until now. Did you complain to the local game store when Age Of Empires went onto the budget shelf, or to the supermarket when the Men In Black DVD went into the bargain bin? Have you taken your PC back because it's cheaper now?
Don't forget that these lower-return outlets ensure the TGC guys can afford to continue working on the next update. Every time someone signs up to a Trialpay/FPSC offer, it helps secure your next
free (in the true sense of the word) set of updates.