Using martingale is not illegal, because it will make you lose money. Casinos would very much love it if you could use martingale, they would even support you during your tries as well, give you gifts and rewards so that you could keep on doing martingale. The reason is simple, if you do it long enough then you are going to lose.
The chance of you going 10+ loss in a row is not as little as people think it is, you may not hit it right away, but if you keep doing it then you are eventually going to get it. This is why casinos very much like it, it has never been illegal in any casino, because it has never made any casino lose any money at all ever, it has always been a profitable thing for all of them so they let you do it.
Yes, that's right, it's legitimate to do, indeed the strategy looks like we will get a bigger win, but it's also worth the risk. Casinos will also be aware of things like this, and this strategy is familiar to our ears, in the ears of gamblers. We have the opportunity to get a bigger win, but the casino also has a great opportunity to spend our balance quickly.
I prefer a steady bet, after all, if luck is on our side, the big win will come to us without any strategy. And does the gambler win more with strategy? Not really, right?