The e-mail by itself isn't the only problem. It's that it's just one line in a list of unethical, illegal or abusive behaviour that's as long as his arm.
Keep in mind - this is also the guy that stole money from the IRS (By funding his previous company with money he was supposed to withhold for employee income taxes... And by not charging taxi/sales taxes on Uber rides in many, many jurisdcitions), from drivers (By dramatically cutting their share of fares - after many of them were locked into long-term leases for their vehicles - leases sold to them by Uber), and who has blatantly lied to the public. (About Uber's background check policies, or the lack thereof. While having the audacity to charge riders a 'background check fee'.)
If running a successful startup requires being a completely terrible corporate citizen, then the valley needs to be burnt to the ground, before it drags the rest of us down with it.