While it's fun to say "just leave," I think it makes more sense for the long-term to try to work it out first. Why? Because odds are, this WILL happen again in another band situation. It's happened to most of us at some point or another. So you should learn how to deal with the issue and hopefully resolve it if you enjoy the gig otherwise. It's a good exercise in conflict resolution and problem solving that you will likely need in future bands. Plus, it seems you're being taken advantage of and you should learn how to break that cycle by addressing the issues head-on before just leaving. In addition, some band members (here, the singer) honestly may think they're not doing anything wrong. That's the way they've always done it and nobody has taught them proper band edicit. Sometimes we think people know better, but they actually don't -- and it may be a wake-up call. In addition, bass players are not easy to replace, so your opinion may hold more weight than you realize in a full band meeting (the best way to do it). You can simply say, there are a number of things you like about the band including a, b, and c, but there are some issues that are outweighing the good parts including x, y, and z.
The end result may be the same...it may be time to move on from this group. But it doesn't mean you won't face similar issues in the future, so it's good to learn how to address them rather than just "walk" every time the balance of power and decision-making gets out-of-whack in a band.