This is nothing new. For example you pay for Aralon about 7$ and then you discover there is also an ingame shop where you buy so called Karma (100 points for 2$, etc) and with that Karma you can buy ingame items. Also a singleplayergame, although for smartphones.
I wonder if new game reviews will even mention this obvious attrocity and integrate it in the game scores (a minus of course if a game has it).
It practically comes to this - now you pay for the demo and then with microtransactions it may happen you pay even more than in the case the full game was selling at some "normal" price.
F2P is already a despicable model that allows grabbing the money even in utter crap as there is always someone who'll buy something just because it's cheap. In some countries there is an ongoing debate if F2P model is actually a fraud and I'm on the side that says it is. If you advertise something as FREE than it should be FREE. If it's not free then say it's microtransactions or whatever, just don't lie to buyers.
In any case, I'm not buying AC3.