> Non-competes are unenforceable in many places, such clauses can be nullified.

Many places being only California, North Dakota, the District of Columbia, and Oklahoma, and even there they're not entirely unenforceable. You'll still get fired, but you probably won't get sued. Maine, Maryland, New Hampshire, Rhode Island, and Washington have banned non-compete agreements only for low-wage workers. There earning more than $34K/yr and in the rest of the US, a contract is a contract, and violators will not only get fired, they're open to lawsuit for breach of contract, though I expect litigating an employee for breach of contract would be exceptional, say if the employee pilfered lucrative clients.

> I'd argue that it's unethical to put unenforceable terms into contracts in order to scare the signee into not doing something they might want to do, something that could better their family's lives, and something that's completely legal.

This is quite the straw man as noncompete contracts are always enforceable through termination with cause, and they are ubiquitous in standard boilerplate employment contracts.

I've worked 35 years in technology under contract and direct hire in CA, TX, NY, PA, DC, MD & NC, and I have never not seen an NDA, which is par for the course in IT (which is not software development), nor have I ever known an employer that tolerated moonlighting. I knew a wizkid Cisco guy, 23yo at the time and already competent with impressive NOC, switch and UNIX sysadmin skills making bank in Austin, took a second full time job for a comparable salary, neither remote. He worked 80-100hrs every week, day and evening shifts, and didn't get caught for 2 years. But when he was caught, he was fired from both locations and lost their references. I doubt it held him back much, and I am sure he has recovered by now, but had to explain 3 years missing from his resume at every next job interview.

I think you're the one confusing the two. Violating a unenforceable contract clause isn't unlawful, but is pretty much breaking a promise, which is definitely unethical.

Are "one job at a time" clauses unenforceable?

I'm not sure what you mean by "unenforceable." In all cases, the job is lost, and that is enforcement enough. It isn't a criminal issue, and I seriously doubt anyone has ever been arrested, but nothing can stop a lawsuit from being filed. Whether it is successful or not isn't the issue. Even if you win a lawsuit, you can still lose your savings and your home. So be careful with your signature, read the fine print, and be resigned to keep your word, and have no worries. Screw around, and you'll at the very least be terminated without a reference for however long you worked, leaving ugly conspicuous gaps in your resume. 9 times out of 10, an employer will hire the less competent but more honest individual. Once you lose your credibility, it's gone.