Unless you have a written contract, you can be fired for any reason or no reason at all. You can quit for any reason or no reason at all. The foregoing is true because Illinois is an "at-will" employment State. There are exceptions to this, but not based on what you've described. I suspect the handbook you referenced has a specific disclaimer stating that it is not a contract of employment.
Your employer may have terrible reasons to tell you where and where not to work. Regretfully, it doesn't matter. If it is a comptetitor, their reasoning is a bit more understandable.
Good luck to you.
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Your current employer cannot really force you not to work at a certain other place. However, Illinois is an at will employment state. Your employer can hire or fire you for any reason or, for no reason, or even for a bad reason. Unless you are a union employee or a member of a legally protected class, such a a member of a certain race or religion and have been discriminated because of your membership in that class, then you have no protection.
The bottom line is this - You need to decide whether keeping your present job is worth it. If not, go take the second job and look around for another primary job when/if you get fired.
As stated , your employer cannot force you to do anything but can fire you for nothing. Decide which job is more important, work there and move on.
I see few or no real legal imolications.
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They can't force you to but they can fire you. However, you stated that others work second jobs. If you are being treated differently than others of a different race, gender, or other protected category perhaps the employer's reason they are giving you isn't the real reason. If that's not the case then you probably don't have any protection should they fire you for working somewhere they don't like. Though it's unclear that you have to tell them where you work in the first place...
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