People asking for penetrable trees fall into the group of people who don't know what they want. You have to give up realism sometimes for playability. If trees and hedges become penetrable, there is no cover at all left on the battlefield from tank rounds or bombs. Strategy would become entirely useless as you could be shot anywhere, anytime and tanks could drive straight through forests unhindered.
The issue with tanks being flipped by trees is a physics engine/collision detection problem. Collision checks are run at a less-than-realtime rate to reduce CPU load. A vehicle traveling fast enough to get inside an object between checks will have "buried" itself inside it; since objects attempt to impart equal force upon each other (ignoring it on the tree's side since it's unable to move), the force multiplies due to the deep impact and the tank is thrown. There isn't really much that can be done about it.
