And you are being paid in experience on a daily basis. Should your employer not pay you? After all, you are getting good work experience, why should the a company also give you money, too?
Now that you mention it, I’m currently doing a lot of volunteer work in preparation for paid opportunities that may come available partly as a result of my volunteer work. Whether I “should” be paid or not is moot — the people I’m working with don’t have money to pay me right now. I’m co-investing sweat equity with them.
In answer to your question, the market puts a price on work (different prices for different types and qualities of work). If a company doesn’t pay you what you’re worth, you’ll go elsewhere. That’s why companies pay for work.