Revenge? Spite? A feeling of importance and control in a situation completely out of their control? Shits and giggles?

Not sure angry human beings are the most rational actors in most cases.

Usually it's more people who get laid off start talking about it on Twitter, then the news media get wind of it and start talking with the laid off employees, and find some willing to describe in detail exactly what went down, send along a screenshot of the announcement email, etc.

If I got laid off I'd talk about it on Twitter too. That's a natural reaction for many people. You want to commiserate with others in the same situation and share useful unemployment and job-finding resources. Hell, a big layoff gets its own hashtag.

Why does anyone talk to a journalist when they don't get anything out of it or advance their interests? People like to tell their story, especially when aggrieved.

There are also legal aspects to doing layoffs (WARN Act, providing return transportation to home country for employees on certain visas[1], plus state-specific legislation like in California). Unethical companies have gone without doing these things in the past, so accountability is in everyone's interest here.

  [1] 8 USC §1184 (c)(5)(A): In the case of an alien who is provided nonimmigrant status under section 1101(a)(15)(H)(i)(b) or 1101(a)(15)(H)(ii)(b) of this title and who is dismissed from employment by the employer before the end of the period of authorized admission, the employer shall be liable for the reasonable costs of return transportation of the alien abroad.

I think if someone gets laid off from their job they should feel free to tell whoever they want about that. Don't you think so?

Here is a better question. Why not do this?

If you got fired, you don't really own the company anything anymore.