Its fairly variant. As far as I can tell (and I'm no expert) the max is either non-existent or so high that i've never encountered it in the wild. Between 2-5% seems common in my experience.

The companies do not have to do this. It is purely optional, though there is a tax advantage I believe. Most "professionals" have the option at their company.

A bigger problem is that the 401k offerings are fairly bad usually. The people picking them are HR staffers who frequently don't have experience in finance, so the plans frequently have murderously high expense ratios. Even to the point where some companies repackage Vanguard funds that you can get on the open market for 10x less.