Personally, I pay very less attention to bonuses. Provided the casino is honest enough to give me my winning and they don't do any shady things with my account or fund them I'm cool. I only use casinos I trust.

There's actually no way to know if they actually keep their word but I believe it's just a marketing strategy.  They may actually give to some customers as you said and I also believe they will give to their regulars. Casinos make a lot of money so giving out bonuses to their regulars shouldn't be an issue for them because that money is just peanuts compared to what they make daily.