As Pie said that put them under the impression that you should be paid for doing something, kids need to learn early that you can't expect to always receive something for helping. Yeah you can pay them, but don't do it for everything they do.
Actually, you should expect to be paid for doing something. My time is not charity. Unless it's very clear that I'm volunteering to help for free, I expect to be compensated for my labor. I'm not making a comment about chores in your house, I'm more talking about how the world works outside of your house, where the 'lesson' would be applied.