Most POSes are running custom Android ROMs which are super locked down for PCI compliance nowadays; the only apps that are allowed are those provided by the POS company, basically.

The POS industry is lucky that custom ROMs are exempt from the upcoming Google lockdowns or they like other embedded device industries running on Android would be screwed…

Here’s the wiki page for PCI compliance, but the jist is in order for a POS to be able to process payment cards, it has to be crazy locked down for actually valid security purposes.