Oh, come'on, they are not a non-profit company !
Dealers/company who have a merchant account
we credit cards companies usualy have to pay
a 4% to 5% fee on each mail-order transaction !
And don't receive the money before 30 days !

Do you realy believed that they would give
unlimited credit to anybody like that ? And
that they would never take a fee when they
probably have to pay one themselve ?!?

As to charging the fee on the transaction to
the customers... Well, that's up to you naturaly. 
But all serious business don't do it (after all, 
there is also fee of one kind or another to pay 
when using other method of paiment. And we 
perfectly know that we are selling more because we 
accept credit card).
Personaly, I would never buy from someone that 
want to over-charge me because I use a card...

Cheers,
Yann Stettler

PS: And yes, everytime someone buy from me using
a card (more than 90% of all orders), the credit
card company take 4% on the transaction.