Yep, if you're far away from the trees and/or whizzing past them at 250kph you're not likely to notice that optimisation, whereas making all the leaves 3D would be a massive FPS killer, so it's an easy decision to make. Because they're 2D, they always have to face a player (otherwise at side-on you'd only see a line), which is why they appear to rotate when in freecam/photomode (also possibly in replay mode).
You may find one day that there is a transitional phase employed later where only critical nearby trees are fully 3D and others are not. Or, devs may choose to improve other things and leave tree tech 'as is'.
If you've got access to the WMD forums there is a 3D Trees thread there where there are some technical posts made by SMS on the subject. I think Bathurst had (or has, I forget) 3D trees at one point.