Force is an interesting word. Definitely unethical to "force" anyone to do anything, isn't it? As for the legality... most of the time there are rewards or penalties to motivate people to act a certain way. People still have the choice to comply or not.
about the only one i could image you could "force" anyone to use would be Firefox - it would be easy to only display your site to FF users, and display a message that says "this site only works with FF - Get it here" to anyone not using FF.. as a matter of fact, there is (was?) a script called Internet Destroyer that did exactly that... as to how Google feels about this? I dunno - I would check with them first.. if you're talking about 'forcing' someone to get one of the 2.0 Referrals, not really sure how you would do that anyway..