In my opinion , If there is bitcoin debit cards then It's for the sole reason that bitcoin is pretty stable and exists for longer time then altcoins and that make it have more credibility then alts , adding multiple unstable cryptocoins could lead to a chaos and you/or service provider could lose money instead of making profits.
I'm not saying the idea is bad , It's good actually , but If bigger services didn't do that then It's probably for a good and wise reason.
No you spend your altcoins to PAY with fiat , so the service provider is getting US , the buyer is spending coins.