Yes, I do understand merchant accounts, I HAVE ONE.
Even if they only receive payments once a month, they are still sitting on your money. By the time you get your statement and start to dispute the charge, they have had your money for no telling how long.
Some merchant accounts do draw interest, particularily those operated by BIG businesses.
I receive interest on monies in my merchant account on a monthly basis, so I do know what I am talking about.