This is an extremely dangerous precedent, and probably violates tabloids' contracts with supermarket chains. Tabloids PAY for that checkout space.
The week after WTC, my local Safeway covered tabloids that had photos of terrorists on the covers. They said some people had complained that they didn't want to see anything prejudicial against Muslims. I objected to the contrary, saying it was immoral not to show people who attacked us and may attack again -- and after threatening to call the offices of the tabloid publishers, I finally prevailed. The liberal idiots at the markets were covering up every issue that was uncomplimentary to Bill and Hill, too. I consider that economic terrorism agains publishers.
I live on a peaceful pastoral island --99% liberal--but it was on direct path of Ahmed Resham had he not been stopped as he drove off ferry from Canada with carload of explosives intended to blow up LAX. You really cannot get out of the mainstream these days, nor should any information be surpressed anywhere.
The ACLU might take up these cases of censorship, and they'll probably prevail if they do.