That is simply not true. While discrimination on the basis of race etc. is not allowed, discrimination on the basis of how a service is used is allowed, thus an ISP can ban a user who is using the service to post child porn. It is just a matter of time before the criteria for this extends to all that the likes of the racist hateful Southern Poverty Law Center deems a "hate group."
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So until internet providers become Regulated Utilities, there is not much customers can do but complain and leave. Now there maybe some room if the phone company or cable company is providing the internet content, but our complaint is with Twitter and Facebook, etc. which are not owned by a phone or cable company. The internet is the highway, our complaint is with the drivers of the vehicles on the highway.