In the US, you have to fill out a form that gets sent to the FBI and IRS if you use over $10,000 cash in one transaction. It's still legal. I think this is a much better solution than banning cash transfers over $10,000.
Form 8300 must be filed and is sent to various agencies for multiple cash payments over $10k as well, which includes, for example, paying for a car or paying for rent.
These laws are old and stem from a time when $10k was equal to around $70k now (edit: someone else points out "Bank Secrecy Act of 1970 sets reporting at $10K which would be ~$80K today"). They've simply never been updated.
