The laws relating to a ban of advertising and attempting to get around that with going beyond simply noting a donor as a source but promoting the source.
You asked if it was an "attack," and I used that word with journalistic integrity. I also referenced the FCC investigation. Those are different parts of my post. I'll address the FCC investigation. The letters were written to NPR and PBS, as though they are responsible for the way all non-commercial stations acknowledge their sponsors. I agree with all of the issues Oxenford brings up, such as the letter lacked specifics or examples, and wasn't based on any complaints or other typical communication. I've addressed his letter in multiple other posts on this board.
NPR's response was that they have always consulted the FCC in the way they handle funding credits. They're always pre-recorded, and the language has been gone over by lawyers to ensure they comply with the law. But how all the local stations handle THEIR funding credits is a completely different matter. Public broadcasting is not a centralized, top-down bureaucracy run by NPR and PBS. Instead you have hundreds of independent local stations that all follow their own rules. Is it possible that some of the local stations have gone beyond the law in funding credits? Sure. The FCC fined WBAI last year for this exact thing. However, WBAI is not affiliated with NPR nor does it accept federal funds. The other examples I'm aware of were either religious stations or LPFMs. But Carr's letter was aimed only at NPR and PBS. So it's hard to know what he's talking about. That's why Gomez called it a fishing expedition.
The other curious part of Carr's letter was the concluding paragraph, where he gives his personal opinion about federal funding. This has nothing to do with following applicable laws. It's strictly his opinion. If he was concerned about following applicable laws, he might start with the preamble to the public broadcasting act, that states clearly that "It is in the public interest" that the federal government support public broadcasting. There is also the last paragraph, that prohibits any government employee from interfering in public broadcasting programming. That's why I say it's not up to the president to decide if NPR News is biased.
Going back to Oxenford's article, he points out that any abrogation from the funding announcement laws would be met with a fine. That's how the FCC has addressed this in the past. But Carr's inclusion of his opinions on federal funding implies that he believes the penalty in this case should be the total cutting of federal funding. That's way beyond anything the FCC has done before, and brings up the recent Supreme Court Chevron ruling about how far an agency can go in its penalties.
How do you figure that recission doesn't apply here? The funds are part of the budget that a President signs into law.
So is the funding for the courts and the congress. The president can't just rescind funding for co-equal branches of government. He also can't rescind funding for independent agencies. It's not his money, and the budget isn't under his branch. The role of the president and CPB is at the core of pending litigation, so congress really can't take this action until that litigation is complete. Further, rescinding the advance funding of public broadcasting breaks other laws that address how congress funds public broadcasting. Those will likely be addressed if the senate passes the house version of the bill.