Yes, I've read Snowcrash, yes we live in dystopian future; at the same time, if I try to be a devil's advocate and simplify it to very basics - if I'm running a two person shop, and one of them is saying bad things about my/our company... I wouldn't really be excited about working with them either, would I? I ran a photography side business for a while, and if my assistant had feedback to share with me, awesome; on the other hand, if they went on Tweeter and criticized our working, what's my motivation to call them up and work closely with them again? :-/
Things get far murkier at larger companies of course, and there's labour law etc, but I imagine there'd always be some tension in speaking negatively about your employer.
(Edit: As for "talking about hiring process" specifically, that may get even trickier; unless you're an HR manager actually familiar and proficient and authorized to formally represent hiring process to prospective candidates, you may cause untold amount of ruckus even with best intentions in describing your limited perspective on the hiring process to people - setting up incorrect expectations or even accidentally prejudicing against the company in any potential latter lawsuit. Just because Person A has had specific experience in hiring, in no way guarantees that Person B will, but it can and has easily been interpreted that way when Person A says "Company A hiring practices are as such").
It seems pretty scummy to prohibit or penalize employees for talking about their initial hiring interactions with the company. Especially if they keep their remarks to the period prior to their officially being hired.
At that point... you weren't an employee. And it's pretty odd for a company to assume they have a right to your silence, covering a period before you had entered into a contractual relationship with them and they were paying you.