They have the right to make whichever minimum bet they see fit, and you can simply choose a website which offers different conditions. So while these changes with minimum bets sound shady and the support's responses are rude, it's not technically scam. Since Nitrogenbetting is owned by Nitrogensports, you can try to contact them on Bitcointalk as well.
I dont see the point on abusing the bonus though since it does still require 30x wager requirement so its out of the possible motives.
I dont see that those set-up when it comes to minimal bets are indeed intentional and as same where Twitchyseal mentioned above this isnt something can be called a scam.
You've been aware and if they are really intending to hook up players to lose then they wouldnt really mind on giving you out some bonus or compensation.