Also, I don't see what's the legal problem with spying on your competitors. I mean, Google and Facebook are spying on their customers, and the law apparently has no problem with that.
They should have because in a civil case like this there is a property called discovery where each side gets to request certain information from the other side. And the other side is required to give it or argue it's outside the bounds of what they can request to the judge, who will decide. In this case Uber had already legally agreed/been ordered to give Waymo all documents in categories this letter clearly falls under, in various ways. For instance one of the more damning given what we know about the letter would be "All DOCUMENTS REGARDING DEFENDANTS’ policies regarding employees’ use of personal computers or other devices while working at or for DEFENDANTS."
A lot is still redacted from the public, but you can see more details on page 7 of their brief: https://www.courtlistener.com/recap/gov.uscourts.cand.308136...
I say this is the most important part of your question, because this is the one area where Uber pretty much unquestionably, intentionally broke the law. Regardless of whether or the allegations in the letter are true, it was a document that they were legally required to turn over but didn't. The head council explicitly made the decision to turn it over to the US attorney and not Waymo so it's not even plausible they missed it.
(I am not a lawyer)
It's not spying, it's doing so illegally. Facebook and Google don't do that. The letter accuses Uber of doing just that, and that's pretty damned relevant in a case where Uber stole trade secrets.
ref: https://www.bloomberg.com/news/articles/2017-11-28/waymo-tri...
An employee was accusing them of doing something highly unethical and potentially illegal regarding information about competitors. Again, that's incredibly relevant.
They will use their knowledge of what doesn't work, and how not to approach problems gleaned from their previous employer to solve your problems much quicker.
Previous employers cost to develop a space laser: 5 years.
Time to re-invent space laser when you already know how to do it: 6 months.
Time to invent: 0
We don't know what's in the letter (yet) but it very well could touch on exactly what Uber was trying to hide, hence the 7.5 million dollar bribe.