The S Rated 1 out of 5 stars
As many have said, THIS IS A BRITISH ENGLISH DICTIONARY! NOT AMERICAN! We do not spell it Optimize, or Realize. If you are going to be a BRITISH English one, get your spellings right!
-ize spellings, though generally not preferred, are not incorrect. Therefore the dictionary does not flag them as wrong.
If you are seeing -ise words flagged, please check your settings, since this dictionary contains a plethora of -ise endings (including optimise and realise).