Some sites that block "+" in email addresses are actually just doing it out of incompetence. My credit union, for example, will actually accept an address with a "+" in it, but nothing will work because some broken bit of web 1.0 plumbing along the way converted it to a space (it shows up that way on my profile page). I wouldn't be surprised to see "&nbsp" on my printed bank statements.

Oh yes, so many websites are incompetent like that.

And of course after registering with foo+bar@example.com they will happily send invoices to bar@example.com