I wonder how many fewer special meals there would be if passengers were charged for a special meal? There are obviously costs involved with having a separate supply chain for special meals, so I don't think it's an unjustified extra charge.
In most places it's illegal to discriminate (including charging extra for things) based on one's religion or ethnicity. IMHO it's also immoral to do so regardless of whether the person is Jewish, Muslim, diabetic, etc. For those that genuinely need a "special" meal there would not be any diffence in the number of these meals provided. Those who simply pick these meals for preferential reasons might pay the extra, do without or simply get militant about it, but that's another story.
On a side note, I remember reading a story that basically said "Want to get served first? Order a special meal." To me this is an abuse of a system that is designed to look after those with special needs based on ethnicity, religion or medical problems but there will always be those that abuse various things because they feel they deserve better than everyone else....