You can use UI to promote use or deter it. A great example of how limiting UI can lead to users getting lazier and not doing something that you don’t want them to do. In PayPal’s case would be depositing your full PayPal Disney money into your actual bank account. Most people that use PayPal frequently would know that most of the time their balances are in random numbers like 593.52 or something similar. When you go to the page to transfer your balance, you’re faced with a page that states your total balance and then the amount that you want to transfer to your account.
I will bet that if PayPal were to add a “transfer full balance” button, the number of people that would transfer their full balances to their bank accounts would double, triple or more. What PayPal is effectively doing is getting many people to leave the difference between the hard-to-think-about-number and the rounded number in their PayPal accounts (which PP will earn interest on). This very simple decision makes the user think twice about transferring your full balance (or just once about not transferring their full balance). You might say… those are some really lazy (or even stupid) users but you have to think about the general population and the way you think when you’re switching between 10 tabs. You’re probably not performing at your best and it’s very low of PayPal to take advantage of that. Shame on them. They should be improving user experience, not hindering it.