"Not only is this none of the bank's business IMO, but it's pretty ridiculous to think banks could be questioning you about your purchases and possibly exit their relationship with you because of this."

Unfortunately, it is the US bank's business due to anti-money laundering regulations and laws enacted by the federal government. Cryptocurrencies are listed as one of the high-risk items, along the same lines like payment transfer businesses like Western Union and Moneygram. If American banks are found to not have practiced sufficient due diligence in stopping money laundering they can get hit with a big fine.

Due to that all of the big US banks now have their own AML department separate from Fraud.