In my opinion, it's fine if a business doesn't want funds that come straight from an online casino for example, as long as they don't call a coin 'forever tainted' if it once came from such a casino.
If you withdraw to your personal wallet and then send the funds to cex.io, they should absolutely be accepted.
But then the casino could do the same and just transfer their funds to a new wallet and then use them again. Just trying to taint a coin for whatever reason creates a whole lot of unnecessary problems.
Agreed. Coinbase's "you're not allowed to send coins to or from a casino" demands are the oldest form of "taint" I've seen, and somehow the Bitcoin community largely accepted that. As if Coinbase has any business checking how people spend their money.