In fact the -ise ending is pronounced with a 'z' sound because of French spelling rules, but since most English speaking people don't learn French spelling rules they don't realise the etymology and hence (not henka) the spelling. The rule in its most basic form is either an 'i' or an 'e' before or after and 's' changes the sound to a 'z'. In a similar way and 'i' or an 'e' before or after a 'c' changes it into an 's' sound; this is why France is spelt France but Francaise is spelt Française, the cédille is there to remind you that the base word has a 'e' after the 'c'. So one could say that the '-ize' ending may have a more Germanic feel to it. In a similar way "metre" versus "meter".
When I was in primary school I was taught basic French, German, Latin and Greek spelling and etymology so that spelling in English would make more sense (not senze!)