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.
Even if they were giving away money, as you said, it wouldn't really cause them any harm as they probably make way too much profit and give away only a very small portion of it to the gamblers, but, they don't really give away money. It could be called money if it was cash balance given and was withdrawable without any requirements.
But bonuses aren't money and shouldn't be considered that, they are just bonuses or you could say, are chances given to players so that they can try and win some money with it, if they fail, it's gone, and even if they succeed, there are certain conditions to be met before they can cash it out.