It all pretty much boils down to money. Some people are unable or unwilling to upgrade their system/device especially if its sole use is gaming. Zup could make trees mandatory, but then he pushes those folks out of his customer base, and cuts down even further on the funds he can funnel back into the game to keep it afloat.
Now if he had the time and capability, Zup might be able to figure out how many subscribers are playing on treeless devices; if the number was low enough, it could be worthwhile to go ahead and make trees mandatory. Frankly speaking, while the loss of too many non-subscribers might hurt the overall playing experience (due to a lower number of potential players to plug into matches); it’s the subscribers that are paying the bills.
Eventually this will all be a moot issue; Unity will eventually upgrade to a point that the older devices are forced out of the equation, and we will all have trees.