It's a question of balance and confidence which is why it's such a hurdle as you learn to ride.
There are good reasons why the lift people want you to ride with one foot out of the binding, if you ignore what the lift guys tell you there's a good chance they'll either refuse you access to the lift or stop the lift until you comply. If you do ignore them you're sending a message that you can't balance on your board and that you don't really care about the other people on the lift and you don't want to be that guy :D
Like oopalley and moremountain suggested you need to do plenty of sliding around with only one foot in the binding on easier terrain and get used to it. Once you've acquired that balance the lifts shouldn't hold any problems for you and of course it's all good practice anyway to improve your balance.
Funnily enough the worst people to share a lift with turn out to be skiers without poles though :D
