I think Bitcoin gambling is illegal because even Bitcoin isn't accepted in many country, so using them with a purpose is totally illegal
Ya their are many countries where gambling is illegal so definitely if you gamble through any source of medium its illegal. Also even if gambling is legal but bitcoin is not then too using btc for the gambling become illegal. So it will definitely depend upon the country and its polices towards it. May be better to avoid if some is not sure about both the aspects of it.
No gambling is not legal. All the gamblings are illegal but people gamble anyway. In my country all types of gambling are illegal and as bitcoin is still decentralized therefore the governments has nothing to do with bitcoin, therefore the gamblers put their money here without any fear and hesitation. I dont know about the other countries. Casinos mean that gambling is legal in those countries.