Sometimes, giving bonuses like that is a sign that the gambling platform is likely going to have another way of getting back the money that went to you as bonus. They might even scam you at the end of the day.
It's not just sometimes, most bonuses always have a requirement (rollovers, odds requirement, minimum bet) that increases your chances of losing and if there's no requirement the reward is small.
I am not really sure but I think most top bitcoin gambling platform should accommodate deposit made with bitcoin
But for sure not all would give you bonuses
A lot of the top bitcoin gambling sites here in the forum doesn't have bonuses but they do have promotions. On fiat gambling sites it's easy to search and compare bonuses but on bitcoin sportsbooks there's only a few and they're not even worth it.