It's just the setting. I didn't find it to be preachy at all, but just a fun story about a silly robot who winds up becoming a hero.
So, the original planet of the Apes should be boycotted because we screwed things up? Terminator? Blade Runner? Running Man? All the other movies that depict a dystopian future?
The movie has two very positive stories in it. The reason the humans are fat was explained in the movie. Not because they were lazy, but because of the effects of micro-gravity and bone loss.
Realistically, the environment should be a conservative issue. There is nothing conservative about waste, over-consumption, trashing things just because you can. It's the manner in how the environmental issues are dealt with that makes us different from the lefties. We were told to be good stewards, but also to have dominion over the land and animals.
I'm as concerned about clean air and water as the liberals, I just don't think draconian laws that inhibit business are the solution, but rather that business is the solution.