In theory yes, they should be allowed for their opinion. However their hold on society is much bigger than the 1 vote they are allowed. With their billions, they can contribute to elect and influence politicians. So while the view to destroy the USPS is well funded, what about the small business or lower income people that rely on USPS as an affordable and needed service (in the middle of a pandemic no less)? Who can they lobby? They just have 1 vote and not billions.

So while in theory I agree with your premise that they are allowed to lobby for whatever they want, the reality is that their influence on society is way overboard and should be limited (limited political contributions!).