Nobody is prohibited from hearing it. They can use their own dime to buy internet access and hear it. Freedom of speech does not include the right to a publically funded forum.
There should not be publically funded internet access in the first place, which the existence of creates this problem. Congress has made no law stopping someone from transmitting pronography. No one has the right to demand that the government(read: we the people) fork over money to allow them to hear the speech.
Look at this way. It's as if the Feds passed a law that states, if a library puts Peggy Noonan's latest book on the shelf to be checked out, it must first cut out Chapter 4 with a razor blade.
Is that something Congress should do? Is it something the Constitution permits Congress to do?
That was the legal question before this court.