This is total bollocks, thought out by half-wits. I'm the first one to object to racist language and stereotyping, not on the grounds of PC, but simple courtesy.
However, the fact is that non-Christians are not offended by Christmas, decorations, tinsel, trees, cards or inflatable reindeer. The myth that they are is put about by racists who seek to divide people of different races and is then believed by stupid people.
This means that stupid employers / managers get hold of this idea, act upon it and people blame the Muslims, Hindus whoever for spoiling their Christmas. Don't play their silly games! Celebrate Christmas - it's no more offensive to non-Christians than Diwali or Ramadan are to Christians.
Of course, as productivity slumps in the run up to Christmas, as people spend work time writing cards, putting up decorations, organising parties - then perhaps the employers have an ulterior motive for banning Christmas!