Don't confuse "legal tender" and "what you can spend". Cheques and plastic are also not legal tender, but as you observe, they are widely accepted. Scottish notes are also widely accepted in Scotland. But you're allowed to refuse to accept payment in Scottish notes (or indeed English/Welsh notes, which are also not legal tender in Scotland) as settlement of a debt (in theory, anyway; in practice, if it comes to court, the court will tell you not to be so silly).