Title says it all; also includes other modes of public transport. Hard to imagine airlines will relax their own restrictions before airports do. It's not clear in the article how TSA arrived at September 13 as a date.
September 13 is the Monday after Labor Day, so they probably picked it as an unofficial end to the summer travel season
U.S. society as a whole is in a weird place when it comes to mask usage. As more and more people get the vaccine, it becomes less necessary for those people to wear masks. But given how many people still are NOT vaccinated, I am not comfortable eliminating mask usage altogether at this point, especially with variants continuing to spread around.
TSA probably decided it's easier to keep the mask mandate throughout the peak summer travel season and then reevaluate it in the fall once kids are back in school and passenger counts will likely decrease dramatically. Plus we'll have more updated information about case counts then.
At this point I am more interested in when the United States will welcome ordinary visitors from Europe again. Waiting for fifteen months now and counting.