This does happen if they have already used this email or credit card on PayPal and created a PayPal account. From that moment on PayPal will require them to login into that account and will not allow to simply use a credit card. This is a security/fraud precaution by PayPal and we have no control over it.
In all other cases users will have a choice of either entering a credit card - or using PayPal account. (If they have never had a PayPal account, credit card entry screen will be the default).
Let us know if this explains the situation.
Dmitry Buterin, Chief Apricot