So that there is at least some possibility of being punished for making stupid decisions. People complain about snipers/etc "playing cod", yet this kind of hand-holding is the exact same thing, worse even. Locking it so friendlies can't steal your vehicle when it spawns is a good idea, that actually was a problem in ren.
If you buy a med tank on Walls from the Ref, then decide to go mine the PP while your vehicle sits infront of the WF, that is stupid. You should feel stupid for doing that, especially if you die at the PP or on your way from the PP to WF. Getting your vehicle stolen is a great way to go about that. If that happened to you, would you honestly make the same mistake again, or next time would you do things differently?
Vehicles probably shouldn't park on the GDI side of the strip to begin with, that's a map design issue.
If GDI has tanks sitting on your strip, your vehicle is going to die before you can get to it anyway, and you shouldn't have wasted money on that tank