licensing should be the same as any computer. if the dev wants to allow you more than one lic per purchase then that's up to them. I would imagine that they would also have to take care of the credit, or send you the file to manually load.
If I had to guess though, it's pay per device for everyone to keep it simple.
I believe each Droid is its own account and the developer has to allow each account to sign into his web access account.
I suppose it will be possible to hack the system to forward it from account to another account at some point, but I do not feel that is what the developer had in mind.
Makes sense. I guess its to cheap most apps its no big deal but it did cross my mind. Like you mention when I buy Office its good on 3 computers etc...