How are cold-calling ceasefires illegal?

I’ve been reading the news lately of Google, Apple, & al. mutually agreeing not to cold-call each other’s employees (and, at worst, absolutely not hire each other’s management). While the later is certainly much more questionable on the legality standpoint, how on earth is it illegal to ask your competition not to cold-call each other’s employees?

People are outraged about this, the vast majority of people being Software Engineers, much like myself - and I absolutely can not understand it.

These actions for Engineers did not stop engineers from switching jobs or getting higher salaries. I see no aspect in which price fixing engineer salaries was involved. Simply the “Hey, stop reaching out to my employees. Let them contact you if they want a job with you.”

And I feel like that’s completely reasonable. The letters from Google in particular were about “Don’t cold-call my employees, and we won’t yours - but if your employees reach out to us or vice-versa they’re fair game.”

And suddenly everyone is claiming that Google is evil and that this is some massive price fixing scandal.

And I can’t even figure out how it’s illegal?