Can I be fired for working a second job?

I have been working the last 3 months at a local Dollar General. My bills got a little heavy so I picked up a second job at Wal-Mart (a Dollar General competitor). Within a few days of Dollar General learning I was given an interview at Wal-Mart, they decided I couldn't work both jobs. One of the managers and lead clerks told me the morning I walked to work that Dollar General was going to either try to make me quit or fire me all because I got a job at Wal-Mart. The following week's schedule I only had one day. I really don't understand why it is a big deal. Wal-Mart knew I was still working at Dollar General and they didn't seem to mind at the interview. When I was hired at Dollar General, I was given no documentation or told that I could not work at another retail store. Neither of the companies have shown me documentation saying I have to work at one or the other. I am wondering how they can do this without legal documentation stating it.

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