They have every right to do it, but that doesn't make it productive. Is github really going to anoint itself word-police and comb through the source code of every project they host looking for words they [currently] consider potty-mouthed? Is that really where we have ended up? Do they have nothing better to do?
What if the word "fail" offended somebody at github? Should they take down projects that have that variable name in their source? (Not asking whether they have the right to do it--asking whether they SHOULD do it)
EDIT:
A couple more things to point out:
1. Think of yourself as an investor who has put money into GitHub. Someone drawing a salary using your money is sending out a message telling a developer not to use a potty-mouth word on the Internet. Do you feel they are putting your investment to the best possible use?
2. As a developer, I now have a new attribute to use to compare GitHub with competitors. I wonder if I have potty-mouth words in my source code. Maybe BitBucket wouldn't put my project in jeopardy because of it...
3. GitHub's own "remove sensitive data" link (referenced in the takedown E-mail) suggests their users use a tool called "BFG Repo-Cleaner". I'm sure they don't think BFG stands for Big Friendly Gun....