My first reaction is always "I never told you a goddamn thing about my email. How'd you get it from swiping my credit card?"
Pretty sure stripe/square/paypal don't share the card's email with the merchants (yet), but even so, it's a bit disconcerting to see the food truck guy "has" my email without me ever giving it out.
Consumer databases are a huge business. It's election day today... think it's an accident that the campaign mailers you got match your census block's demographic profiles? http://www.esri.com/data/esri_data/ziptapestry Normally only the credit card companies were privvy to this data collection. It would be poor business decisions if selling these profiles was not part of stripe and square's business projections.
The solution is to put the cursor in the card number box and hit delete, but this is not obvious in the UI.
I am not aware of Mayday users raising the concern that their details have been saved.
Our audience is heavily from the tech community, and people like seeing the latest technology used on a site. I remember at least one email coming in to that effect.