People gamble on credit all the time, I do agree its a no no but it happens quite often people spend what they dont have.    Really dangerous because this is the path to addiction even possible ruin.  Its far easier to stop when the money gambled was excess and part of a budget for an evenings play anyway.

I also think borrowing is a bad mind set and sets up a possible failure in confidence and clear thought that can sometimes help get an edge in a game.    

I know credit cards generally ban gambling and that is a good idea but I've heard of people mortgaging their houses and all sorts only to gamble their money.  Not just casinos but read a long case of gambling on shares and some people take on various risks really unadvised on credit when they would think twice with their own money.   There is a general law in many places to responsible lending for this reason I think