I really do think that they want people to play less TFT. The Orb was pretty much a guide last time, - 5 matches in a day, is enough. I'll assume it's to be treated the same way now.
Speculations as to why? The game was solved fairly quickly last time... they don't want people to learn it too quickly.
Or, perhaps, they expect play to rise in pre-season anyway. (Lots of players get depressed and disgusted with SR for the first couple months, and turn to ARAM, RGM, and probably TFT this year.) (Not here to argue about why people get upset in the early season, or whether it's a lot of people. It's a phenomenon that afflicts a number of players annually, as evidenced by complaints here, other forums, etc... whatever it is, I'd expect those people to be passing time on TFT while SR straightens itself out, and maybe the administration decided that folks didn't need further encouragement.)