I agree that it's odd for a building as lackluster as the May Tree, but since they do this for all evolving buildings, I think it's part of their evolving bldg revenue strategy. The players who spend real money on this game in amounts that make it so the rest of us can play at a reduced rate or for free frequently have more than one evolving building at max levels. I don't think Inno is opposed to multiples of these max level evolving buildings, they are opposed to players being able to get them without financially supporting the company.
What I don't understand is why they haven't made a way for newer players who are heavy spenders to evolve those older buildings they can craft in the MA. I know it would be expensive (when they sold artifacts outright in one promotion, the cost/artifact was outrageously high) . Still I expect they have newer players who would pay diamonds to, for example, craft artifacts to evolve a Fire Phoenix. If I had known ahead of time in the first event that gave the Fire that it would cycle back around, I'm pretty sure I'd have used up my entire annual entertainment budget to get a second at lvl 10, lol!