I'm pretty sure Skrill doesn't prohibit crypto-related transactions on their platform because they actually allow you to add funds to your Skrill account using Bitcoin and Bitcoin Cash according to their own FAQ (
click for link). If you really want to say something about your transaction, just make it short (and don't say anything at all in the case of PayPal).
Since they don't allow you to withdraw cryptocurrencies as far as I understand, it wouldn't be surprising if they wanted to prevent people from doing crypto-to-skrill exchanges or if they put barriers to bar most of their customers from these kind of trades, because that would mean one party to the transaction could use cryptocurrencies as they see fit, and a lot of these companies are starting to become nosy how people spend their coins.