You're not barred from investigating someone or using investigators to do so in a trial, but the prohibition here is about the effect created by the investigation (just like it's the effect of 100 phone calls a day that constitutes stalking, not using the phone itself). Legal proceedings have a well developed decorum/civility to them that the judge can enforce at her discretion, including criminal penalties.

If Uber/Ergo had limited itself to not interviewing the plaintiff's family, friends, and acquaintances, they'd probably have been fine; but going around to everyone you know and asking a lot of dark questions is pretty clearly going to intimidate a plaintiff and chill other potential plaintiffs, and that's obviously the point--especially when interviewing is unlikely to surface anything not already found through records searches and the discovery process.