I can't speak for the US, but in Germany, transactions between countries over 10k€ must be reported to the Bundesbank. There are similar limits for inner-country transactions with businesses, especially financial ones. I assume the USA has similar rules. So small cash transaction are fine, but as soon as larger sums are involved it's going to be very problematic.

Since crypto currencies, nearly by definition, don't care about country borders and the mixers don't trace the amount put in by each user, they almost certainly allow you to circumvent money laundering registration requirements. It's even worse if they frame the mixer as financial institution, in which case it directly violates its reporting requirements.