They probably had a lawyer advice them on which threats were substantial, and which ones weren't.
The RIAA is famous for sending out threats on anyone or anything that remotely does anything they disagree with. They don't necessarily even have a legal argument to win the case. But they have their lawyers write up nasty letters and emails to coerce you into doing what they want.
Without a lawyer of your own, who can sift through these C&D letters... you're pretty much defenseless.
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More realistically... the Youtube / Tumbler approach was most likely "Ignore the Letters, and hope for the best". These kinds of letters aren't necessarily threatening legal action.
