Yes, I was asking as for the legality side of it, if in the US it is required for the platforms to enable KYC even for crypto-crypto only platforms
The law isn't clear on that yet AFAIK.
USA regulators removed any ambiguity a while ago.
FINCEN repeated this past may that crypto-only exchanges are subject to the bank secrecy act, which has KYC reporting requirements at $3000, $5000, and $10k thresholds:
To address these and other issues, on March 18, 2013, FinCEN issued interpretive guidance on the application of FinCENs regulations to transactions involving the acceptance of currency or funds and the transmission of CVC (2013 VC Guidance). The 2013 VC Guidance described what CVC is for purposes of FinCEN regulations, and reminded the public that persons not exempted from MSB status that accept and transmit either real currency or anything of value that substitutes for currency, including virtual currency, are covered by the definition of money transmitter. The 2013 VC Guidance also identified the participants to generic CVC arrangements, including an exchanger, administrator, and user, and further clarified that exchangers and administrators generally qualify as money transmitters under the BSA, while users do not.
this is why binance gave american customers the boot---so they could retain unverified accounts on the exchange.