Actually the car analogy (while not precisely identical) is a good one. People think just because something is virtual and potentially copyable, it is somehow less a business property / asset than a real life item such as an automobile. To answer your question-- no, I wouldn't expect you to re-buy the same car over and over... but that's not what's being discussed is it? What's being discussed is buying something-- and expecting to be able to copy it from grid to grid to grid. In RL if you need three cars... you're going to have to buy three cars.
Now I will agree it would be very nice to be able to SECURELY transfer my personal inventory from grid to grid. For example, if there is a virtual car that I created and really enjoy driving, it would be nice to be able to visit any grid I wish and still drive that car. And to an extent I can do that-- with my own creations-- if the script engines are compatible. But if I take that car to a grid owned by an unscrupulous, unethical "sim god"... that car can be stolen as easily as one can say "misappropirated".
That's the problem. I think all of us would enjoy the utopian dream where all grids are 100% compatible, stable, functional, with full security in place and everyone honest. But we have to live in the real world.