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Yes, it is legal to condition further employment on the signing of a non-compete. The good news is that California courts do not enforce non-compete agreements except in certain very limited and inapplicable situations. So, if your employer tried to sue you in CA court to enforce their non-compete, you would be able to get that lawsuit immediately thrown out.
Now, there is one caveat. Although CA courts won't enforce non-competes, MN courts will. So, if your employer attempts to sue you in MN and gets a judgment there, something called the "rule of comity" would generally require a CA court to honor that judgment and enter it in CA court so that it could be collected upon you here. The way to defeat this is to seek what's called a "declaratory judgment" from a CA court first. This is essentially a preemptive judgment in which you get the court to declare that the agreement is unenforceable. If you get that before your employer sues you in MN, you can stop the MN lawsuit from proceeding.
Now, all of this isn't exactly what you asked about, but it's relevant to your question. Your employer can force you to sign the agreement as a condition of further employment. But even if you sign, there are avenues to prevent it from ever being enforced against you. Those avenues would cost money for you to pursue, but they exist nonetheless. Also, it bears mention that very often employers either never find out about a breach of a non-compete or if they do find out decide not to sue because it would take too much time and money to do so.
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