I feel you but when I created my post I wasn't even thinking about white reggae artists. Here in America a lot of black people wear dreads without any connection to Rastafari; simply because it is a natural hair style.
Now when it comes to white reggae artists and there are some decent ones out there I have to admit... but often what happens is that they introduce reggae to a new crop of Europeans who get into it through them but are not limited to one artist. They want to hear more just like the way Pandora works. If you truly love reggae you might be more attracted to a certain style but even then you're going to be somewhat open to other forms whether it's roots, dancehall, or other.
Often these white guys will perform classic reggae songs with a band. In my hometown we just had our annual reggae festival in Dayton, Ohio. Sizzla didn't come to Dayton, he was near my cousin in Canada. And Sizzla probably isn't coming to Dayton anytime soon. But who most likely organized this event?
He wasn't the only act though so in other words he basically put food in the mouths of other artists who were also in the line up and of course everyone had the opportunity to sell their music. So this is just one, very recent (last weekend) example, of how appropriation can open doors that our people can walk through. I'm not saying that what you're saying can't also happen or isn't true, but I know there are other cases where it is mutually beneficial. Now... if they were to somehow take it over or do to reggae what Europeans have done to rap then that's on us not to allow that to happen. But that's not even about who's performing it but rather who's producing and distributing, agents and labels, etc.