It's not that. They felt they did not have the legal authority to do this, and that it would be struck down by the courts.
That's done in politics all the time. Their explanation was just a smokescreen. It's politically dangerous as a Republican to look like you have compassion for convicted criminals.
That's a pretty terrible way to run a government. When you elect someone you expect them to follow the law and do their job.
Not just do anything they want and say "it's fine, if we are wrong the court will fix it".
Whether or not it's terrible, it's the system of checks and balances that was envisioned by the founders of the United States. It's exactly how our government has (and does) run for its entire history. The courts exist for this purpose.
"Follow the law" is not cut-and-dried. Statutes can be interpreted any number of ways, and very little common law is set in stone. Even common law created by the Supreme Court (e.g. Roe v Wade) it incredibly controversial and could be challenged again.
The courts are there in case of mistakes, they are not intended as the primary way for government to function.
Do you realize just what you are advocating for? Every little police chief will claim any road he can, until a court tells him to stop. Every property inspector will levy fines for parking tickets until told he can't. Nothing stops me from setting up a toll booth at the end of the road, until the court tells me to stop.
No, this is not the way. People should, on their OWN, know what they should and should not do.
The Republicans felt this was a power grab by the FCC, you would have been happy if in fact they had grabbed that power, but that's because you like the issue at hand. Next time it will be for something you don't like.
So in this case, the Democratic FCC members were willing to take a risk because they thought the potential benefit (giving prisoners affordable access to telephones) is worth it. The Republicans did not, because they are, in this scenario, comfortable with the status quo.
But for most of the time, lawmakers should try to only pass laws that are clearly legal.