Square lets merchants spam you without opt-in if you get an email receipt

This seems pretty standard practice. Enter an email address anywhere on the web and the recipient takes that to mean "I like having my time wasted, please send me spam".

Square needs a fine very similar to Twitter for this.

Let us all report them to the FTC or whatever because this behavior is unconscionable!

So you enter your email address for one purpose (to get a receipt) and they use it for another (to email you adverts).