By banning all religious symbols they are being discriminatory against religion vs. people with no belief, while not necessarily illegal, it could provoke a very negative reaction and possibly challenged in court. People have a right (free speech etc) to express their beliefs. I can't say for certain how it would pan out, schools here in the UK have tried to ban religious items such as Christian chastity rings on the count of them being jewellery, which is banned by school policy, but since the ring is a sign of belief, it was taken to court as discrimination against Christianity (religion) and the girl inevitably won since the school indirectly banned religious expression.
The act of banning offensive imagery is an act of company policy, much like banning nudity (Age ratings excluded). For the most part, when people buy and play the game, they agree to follow the terms and conditions, if such imagery is banned in the terms and they do not agree with it, they (the person) withhold the right to not continue with the service. Additionally, in the terms, it will probably state something about user uploaded content which may or may not contain offensive imagery and you can not hold the company responsible for your perceived offence. If you choose to ignore the the company's policy, the company withholds the right to terminate the service provided to you. Simple as that. The company can do what they like as long as it follows Law... preventing people from expressing their beliefs is a fine line, even through imagery.
Bottom line, they can do what they like, if people don't like it, their choice, they can leave.
