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It's in the top 10 coins by marketcap at the moment, we already supported its "ancestor", plus it's clearly not a security, so it was was a relatively easy decision to add it.

Support for a lot of coins is being delayed till we can separate out US/non-US customers to avoid potential problems if the SEC decides they are securities.

Why do you say it is not a security, is there some guidance out there from SEC or the like?

Yes, there's been plenty of guidance, but it's kind of obvious on the face of it: there wasn't any money raised for bitcoin or the bitcoin cash forks and there's no common enterprise managed by a selected group of people for profit of investors (no investors).