Are "vehicles" bugged or is it as designed?

Hello,

I searched here, I googled and really found nothing about this issue.
With WotLK and the introduction of "vehicles" (some types of mounts included), commands as follow and assist (anything involving target player) stopped working when "mounted". A typical example is Occulus after first boss.

My question is:
Is this a bug, or "as designed"?

Thank you for your time.