Because cannabis is not federally legal most banks won’t allow you to purchase things that aren’t legal. So the dispensary only takes cash so there is no record of you buying something federally illegal. To make it easy they offer to run your card as if you are at an atm. Any time I’ve bought from one you don’t see you bought from a dispensary, it appears as an atm. Atms you can’t take out odd amounts.

Most banks will not open up bank accounts and offer merchant services to dispensaries and won’t until its federally legal.