With that logic maybe banks and credit card companies should start skimming 30% off each transaction. [EDIT] They should also strong-arm you to prevent you mentioning cash as an alternative.
Credit cards do already skim a percentage off every transaction you make with them and, until very recently and an act of congress, actually did restrict vendors from offering a separate price for cash transactions. They were perhaps not so bold as to try and claim 15-30% of the gross. Notably stores accept (or refuse) various credit cards in part due to their percentage of the take, with AmEx being notoriously higher than others.