Regulation should step in and make it HURT to use the lower rtp.
Maybe force them to have a pop-up with some info for the player.
"Hello player.
We at x-casino has chosen to offer a version of this slot that makes us more money.
Instead of 96% return to the player, aka you, this version only gives back 94%.
If you want to have a better chance at winning while playing this slot, here are some casinos that offer the non-reduced rtp version of this slot. X-casino, x-casino, x-casino, etc.."
And voila, no site would use a lower rtp-version ever again.

/QUOTE]
Hey unregulated casinos with suspect software, have a field day.
The fact that certain uk casinos for example lowered the RTP versions is not greed but the need to actually survive/make a profit.