No specific reason needs to be given. And this is important. Neither side should be beholden to the other. Employees are not beholden to their employer to stay any length of time, and neither should employers be beholden to the employees to keep them employed indefinitely.
Now, no matter what, there is going to be a power imbalance. Currently, the power lies mostly with the employers. Employees are generally replaceable. You can find someone else to do a similar job.
So its symbolic in one sense - sure you can quit, but you can't do it without fallout.
And most non-competes are unenforceable as it prevents people from making a living.
The deets may vary around the edges depending on the state (excluding CA which has strong anti-non-compete)
/* I used to practice employee-side employment law.
As usual, there's a system in place and lofty rules about 'right to work' but it functions only for the well-off. The entitled if you will.
The vacation/sick pay issue is more of a grey issue as it's mostly a matter of company policy than any legal issue.
While there are plenty of workers who don't really have any recourse, I'd imagine highly paid tech workers can afford enough of a lawyer's time for them to draft a scary letter.
If your payroll damages are low and your case is solid, you might find a bleeding-heart lawyer to help you because attorney's fees are often included in the employee's damage award.
Though, in some states punitive damages may be available as well. (Sadly, not in Washington State where I used to practice)
>[A]n employer may terminate its employees at will, for any or no reason ... the employer may act peremptorily, arbitrarily, or inconsistently, without providing specific protections such as prior warning, fair procedures, objective evaluation, or preferential reassignment ... The mere existence of an employment relationship affords no expectation, protectible [sic] by law, that employment will continue, or will end only on certain conditions, unless the parties have actually adopted such terms.
http://online.ceb.com/CalCases/C4/24C4t317.htm