> if you chose to send mail using a "private company", it's that's company's "free speech right" to inspect your mail and throw it away if they don't agree with the contents.

That's somewhat true currently.

If you ship mail or packages with UPS, FedEx, Etc they are allowed to open and inspect what you send. The USPS is not allowed to do that, not without a warrant. When people ship drugs via mail they use the USPS for that reason, that it's more difficult for them to obtain warrants to discover suspected drug shipments while a UPS or FedEx will happily open any package they suspect may not be legal to send.