Yeah but customers may have little trust
in a company that keep changing from method
of paiement : after all, they may be doing it
because they keep being kicked-out.

Also, many users won't have the time nor the
will to keep signing-up account with whatever
is the "paiement method of the week" for this
or that dealer.

As to your explaination about interest on the
money that such a company get... you are in
part wrong : when you transfer money to your
paypal account (for example), even if it's
removed immediatly from your bank account, it
doesn't mean that they receive it immediatly :
it's your bank who is delaying the transfer
of the money and that's how _they_ gain money.
So it's actualy not paypay who is gaining
interest but your bank.
Why do you think it takes such a long time to
do wire transfer to another banks ? Such
transaction are fully automatic and processed
at least once per day. If it takes more than
the next business day, it's because your bank
is "pooling" the transfer for a couple of day
and gain interest on it.

Cheers,
Yann Stettler