Buying Gas on CC sites

I just wondered, in this day and age, when I want to change a gas bottle, why can't I use my debit card?  After all, I can pay my site fees by card.  They always specify cash.  I hardly ever carry cash (certainly not more than £20).  It I have to go out to get cash to buy a gas bottle, it could possibly cheaper for me to get gas out and about!

Does anyone feel the same way?

David