How about showing a bit of respect for Commissioner Pai, who is not only a very smart man but is leading the charge for something that everyone should want: transparency at the FCC? You may not agree with him, but don't insult his intelligence.
Besides, he happens to be correct. Title II regulation would be horrible for the Net and would harm Net users worldwide by paving the way for nations to block, throttle, censor, and overcharge.
That behavior does not look like the ISP's equipment is altering messages from the remote server, but rather as if it is redirecting the session to a proxy server -- most likely for the purpose of stopping outbound spam.
Such a proxy can send the mail onward in encrypted form once it's checked for spam. If this is the case, and there is encryption or even just physical security on the inbound connection, the privacy of the message isn't compromised.
It's sad that such measures are necessary, but they are -- not only in public hotspots but in situations where a new user might sign up for new service from an ISP and then send boatloads of spam until the account is turned off. (This is happening a great deal nowadays.)