If all of the major ISPs launch their own email competitor and then go on to block Google, would you be singing the same tune? "It's okay because AT&T, Verizon, Comcast, and Charter are private institutions and are not obligated to do business with Google"? What if it turns out that the AT&T CEO is a Trump supporter, and he decides that anyone who has AT&T as an ISP will no longer be able to access any news site other than Fox News? "It's okay because AT&T is a private company and has no obligation to do business with the Washington Post"?
The entire argument for net neutrality is based on the premise that sometimes even private companies become so big that they become very similar to 'public utilities' and it is absolutely in the public interest to force those companies to not arbitrarily filter whatever they feel like.
You can argue that Cloudflare, or a bank, or whoever doesn't fit this definition, but you can't both be for something like net neutrality while simultaneously spouting this argument that "private companies can do whatever they want". We have literally centuries of laws that specifically say that no, companies cannot do whatever they want just because they are private.
