It requires that you give them your bank account details.

The stated reason is because you can call premium services and run up a charges even with a free service.

Yeah this, and you need to give them your full address because you have to enter a code that is sent via (physical) mail. Also it has the "Datenautomatik" enabled by default where new data packages are bought automatically when the limit is reached but you can turn this off (it's on by default).