A few reasons off the top of my head:

1) Most people accept them because they never actually read their contracts fully.

2) They are told that it is a non-negotiable condition of employment and they really want the position.

3) They are aware that non-competes are unenforceable in their jurisdiction (assuming they are in a jurisdiction where non-competes don't stand up in court).

4) They assume the company is unlikely to spend the money required on lawyers to actually come after them if they violate the non-compete in the future.

5) When hiring you, the company verbally insists up-and-down that they won't enforce the non-compete, and you believe them

I find this too. When we were acquired we (group of employees) had lawyers come in (to a bar) to go over the agreement the new company required we signed. The lawyer basically said not to sign it because they could probably enforce it. Ultimately the company made it seem like we were making a big deal out of something they would never try to enforce but still insisted everyone had to sign it. A few people refused to sign. Other signed and then left for competitors anyway. A majority signed it and still work for the new company.

1) This makes sense as an answer, I don't understand why people wouldn't read the contract for something that they rely on for their livelihood though.

2) Something like that should be a dealbreaker. No matter how much you want the job, it's effectively saying that you can never leave.

3) In the case of the letter here, he tries to negotiate it after he was fired. If he knew it was unenforceable, then he could have quite simply said so.

4) On their part this is a stupid assumption. You should always assume that a company is willing to back up their threats. Especially a company the size of Amazon.

This isn't to attack your answer, just to put my views on those points.

"2) Something like that should be a dealbreaker. No matter how much you want the job, it's effectively saying that you can never leave."

Based on your username I'm going to assume you're not used to conditions in the US. In my experience, working for 2 megacorps and a startup, this is pretty standard. So if that is a deal breaker good luck finding a job without moving to California. I think I read a story on HN a few weeks ago that said Jimmy Johns was enforcing non-compete agreements. The last one I signed basically said the company owns all work I do, even work I do in my own time (OSS, side projects, etc). I also am required to get permissions to contribute to OSS projects should I want to do that. I also cannot work for competitors (which is like everyone in the industry). I signed it because I don't think it is enforceable and I don't think the company would waste resources trying to enforce it on me.

There needs to be a federal law limiting the scope of these agreements. Preferable modeled after the CA laws.

Because you're young and enthusiastic, because the job sounds awesome and because you don't want to employ a lawyer every time you apply for a job?

Sure, it's a bit naieve not to read a job offer carefully enough, but on the other hand you'd kind of assume a company as large as Amazon, employing so many knowledge workers, would not offer their applicants extortion rackets disguised as job contracts to begin with.

A number of new-hire engineers at Amazon are straight out of college. Looking at it recently, no numbers purely anecdotal, this crowd is being dominated by Asian immigrant graduates.

These people are students who just finished their graduate or under-graduate studies and have most likely taken huge education loans in a currency which is weaker than the dollar. In that situation one really just wants a well paying job at a company which is recognized by the general population (especially your parents and peers) as a good one. Given that, the person signing the contract is unlikely to give the non-compete too much thought. And even if they did, verbal rhetoric from the HR is good enough to lead them to signing.

I wouldn't if I ever take a job somewhere where they are enforceable. As it is, it acts as a minor warning sign that the company has asshole lawyers.

Well, if it's the only work you really like to do OR if it's the only job that pays 100k a year?