Do you think they don't check the source of the funds? They do, as well as the banks involved in both sides (since it's also their job).
Do you think Kraken doesn't do that while it has a Money Business registration.
Why do you think they all have paragraphs about AML on their ToS,
If we compare the situation with banks, then in my country the receipt of funds over $ 9,000 is more likely to lead to a blocking of a bank account if I am not registered as an entrepreneur. Then the documents will be transferred to the tax authorities and they will conduct an investigation.
The situation will be similar with cryptocurrencies.