It is certainly unethical service but I would not call it illegal, as nothing is stopping people for having multiple accounts, yes even hundreds of them with different IP addresses.
There is no way you can prevent this in the same way like you can't really moderate and prevent spam in bitcointalk forum.
This services will grow if candies and offers from CoinGecko and CoinMakretCap have bigger value.
I wouldn't tag anyone for doing this, but that is just me.