I think this is very different from Facebook or Twitter skew feeds in favour of some political cause.
First of all they are essentially public utilities, they shouldn't regulate speech as long as it's legal.
Secondly they do it without people's knowledge. David Barret's embarrassing email is completely honest and open.
Hard no. There is no legal nor moral basis for this. Absolutely not.
They can control speech however they want in their realm. If they want to censor, it's completely in their right.
That being asked, while I agree with the message, I don't believe sending it was a good idea. It likely won't move the needle at all (most everyone has made up their mind), and potentially alienates Trump-supporting customers.