I've dug up some information on 5-card-21 paying 2 to 1 and 7-7-7 paying 3 to 2. Which shift the HE .2% and .01% towards the player, respectively. However I could not find anything for this flat pay bonus. It seems like there might be a player advantage if betting the minimum with perfect basic strategy, and perhaps a bigger advantage if there were an adjusted basic strategy for the ruleset.
...5-card-21 (as opposed to 5 card charlie) pays $50 (10 to 1 on 6D min. or 5 to 1 on DD min.)
7-7-7 pays $25 (5 to 1 on 6D min. or 2.5 to 1 on DD min.)
Suited 7-7-7 pays $75 (15 to 1 on 6D min. or 7.5 to 1 on DD min.)...
If you have a 5 card 21 and the dealer makes 21. You get the $50 bonus, and your blackjack bet pushes.
They don't have signs advertising the rule at all, I have called them on telling newbies that on doubledeck before though.