What do they do with a very profitable trader? Can they kick him out like a casino? Or do they just suck it up because the rest will lose with them? If you swing trading the spread isn't a problem specially with this volatility...
No, I've never heard of a profitable long-term trader being stopped from spreadbetting. Of course, the profitable trader pays spreads and over-night charges on increasingly larger positions, so they're a profitable revenue stream for the firm and they certainly haven't been forced to migrate to a competitor.
The only profitable spreadbetters I've ever heard of being barred are those suspected of short-term arbitraging. But that's not because they're profitable as such, its because they make it hard for the firm to manage their risk exposure in real time. I suppose you could say they're akin to card counters in a casino.
if you calculate spread as execution cost and compare it to underlying products it doesn't make sense. Ok it can have sense in several markets, but you will find other issue - chart data. A little longer topic - you can simply compare the chart on futures and CFDs for the same product. You'll see what's going on.