"I can't think of a any reason except manners. Depends on the circumstances. I wouldn't choose a kasbah as a site for my soapbox.
" - Amos

Neither would I. It is quite possible to engage followers of Islam (and I have) in Canada, The US, or the UK, without subjecting myself to the worst aspects of that faith. (It is also possible to be polite in stating one's objections to religion, but it seems often just stating the objection is considered insulting to the believer.) I assume you are implying that Christianity (I may be wrong) is a far kinder and less dangerous set of beliefs, and I should be grateful to be 'allowed' to get up on my soapbox in consequence. That speaks volumes.