Mixers are not prohibited by law.
In the US, a money transmitter license is required as mixers are classified as a "money services business." No mixer is, as far as I know...
https://www.justice.gov/opa/press-release/file/1574581/download18. Based on my training and experience, I am aware the Bank Secrecy Act requires
anyone who owns or controls a money transmitting business to register with the United States
Department of the Treasury. See 31 U.S.C. § 5330(a)(1). I am further aware that federal
regulations issued pursuant to the Bank Secrecy Act define a money services business, which
include money transmitter. 31 C.F.R. § 1010.100(ff)(5). Money transmitters are defined
broadly to include anyone who accept . . . currency, funds, or other value that substitutes for
currency from one person and . . . transmi[ts] . . . currency, funds, or other value that substitutes
for currency to another location or person by any means, as well as [a]ny other person engaged
in the transfer of funds. 31 C.F.R. § 1010.100(ff)(5)(i)(A)-(B). MSBs are required to register
with FinCEN, a division of the Department of the Treasury, unless specific exemptions apply. 31
C.F.R. § 1022.380(a)(1). MSBs are required to establish and maintain anti-money laundering
programs, to detect and report suspicious transactions, and to collect certain records of customers
and customer transactions. I am further aware that bitcoin mixers or tumblers such as
ChipMixer are considered to be MSBs under federal law. See U.S. Department of Treasury
FinCEN Guidance, Application of FinCENs Regulations to Certain Business Models Involving
Convertible Virtual Currencies, FIN-2019-G001 (May 9, 2019), at 19-20.
Sorry to put it so bluntly but anyone advertising a mixer that doesn't have this license is advertising an illegal business (if the business is servicing US-based customers, anyway).
Funny, its actually been the case since May 2019, but either nobody here knew this or the ones who did ignored it.