Towers are already upgradeable, why should they gain experience and upgrade for free?
Because if there is no AI on a team, chances are you run the risk of towers not upgrading at all... and seriously.. when after level 7 do you still even give towers a second thought?
Yeah towers should not upgrade themselves but should give slight (like 50) experience.
And yeah too, I just upgrade the 1st Health/Rengeneration for the buildings then never touch them again.