Asking whether bitcoin casinos are legal is like asking whether is cannabis legal or not. Well, it depends on the specific laws of your country. If your country requires licences for them to run and to comply with anti-money laundering and know your customer laws and they don't then they're probably not technically legal or allowed to operate in that jurisdiction. I would check your own countries laws and what they require for an online gambling business to operate because nobody can give you a concrete answer without knowing where you reside.
Bitcoin casinos are clear in most countries that it is illegal, because in any way all forms of gambling have the potential for crime, especially facilities for money laundering, but even if there are countries that legalize bitcoin casinos, only a small portion of the countries in the world.