Christmas trees are legitamently valuable tactical tools, actually. They block line of sight but not fire or movement, so you can use them in windows to provide protection from snipers without preventing you from using it. You can hide barbed wire or mines under them. You can force enemies to enter a room before being able to know where any possible threats are. Etc.
I reckon thought that the change is more aimed at people inflating their score/engineering assist count by endlessly building christmas trees, than it is at people using them too effectively.