Along with Civil Asset forfeiture mentioned by others, there are also:

* Know Your Customer laws,

* mandatory Suspicious Action Reports of all transactions over a certain value,

* anti-Smurfing or anti-Structuring laws that against structuring deposits to be under the threshold of SAR reports,

* a whole host of foreign transaction laws

* and plenty more...

So yes, prosecution of use of cash is very alive and healthy with people getting prosecuted for all of these crimes related to use of cash. And this is before any prosecutions that establish direct use for terrorism or drug cartels.