I never got charged in any rl shop either for trying out clothes
So tell me, is there a specific reason, other then trying to make profit on selling demos?
The point is the commercial process is not the same in SL and RL. So, I'm not quite sure the comparison with RL shops can be an argument. In SL, for obvious reasons, you need demo items while you don't RL, where you simply try out the acutal product. And a demo pack requires some specific work of its own (such as applying the "demo" label onto a skin or placing a box on top of hair, etc).
This additional work may possibly explain why Demos are dollarbies.
Just my two cents