Are you sure you didn't violate their TOS? Lets assume your documents are legit, are you from one of the countries highlighted below?
(...) including the United States of America (and her dependencies, military bases and territories), Australia, United Kingdom, Estonia, Netherlands or other restricted jurisdictions ("Restricted Jurisdiction") as communicated by us from time to time. By using the Website you confirm you are not a resident in a Restricted Jurisdiction.
Although OP already confirmed not residing in a restricted jurisdictions, I would like to say a few words about this. Bookmakers always use these terms to their advantage - all bookmakers. These rules are only enforced in case of dispute (about withdrawals) and the bookie always wins
If OP was actually from US and got (only) his deposit back after SB found out, this is technically ok. But then again, if OP lost some of his deposit and then SB finds out, that he is from a restricted jurisdiction, he will not get his (full) deposit back. So this rule only ever works one way to the advantage of the bookie. They will take money from losing players, even if they were not allowed to play on their site and can bully people with their T&C when winning.
Sportsbet knows that some of their customers (who are active in this forum) are from UK or Netherlands - do they block their accounts ? No. Do these users knowingly violate the terms of Sportsbet ? Yes.
Afterall this a weird and often times shady business, but something we have to live with to a certain extent and as I said, it's the same everywhere and not only at Sportsbet. And we can always choose to not be part of it, it's not mandatory to gamble fortunately
If $3,644 is a small amount for you then you're living in a different planet.
It's peanuts for Sportsbet
And the reason, why I don't think this is some sort of a scam.