They are not recommended for isocyanates because of possible mis-use not because they are not effective against them. What was mentioned before about people leaving them on the bench for weeks or even months or trying to use them with a beard or an ill-fitting mask that allows leakage or any number of other things that people WILL do is why they are not approved for iso's. They most certainly will protect from isocyanate exposure and there are people here who have used them successfully for that purpose for years, the key is to USE THEM PROPERLY! The manufacturers are quite aware that when these things are sold to the general public that the vast majority are not trained in their proper use and a great many will simply ignore included warnings and instructions so the manufacturers in order to protect their arses from legal problems will simply exclude their use for anything as immediately and severely dangerous as iso's. With a properly fitting mask on a clean shaven face and properly checked for leaks and FRESH filters plus a fairly well ventilated painting area these things will provide ample protection. Some guys will just grab a mask that has been lying around open for weeks, not even think about checking for proper fit and then spray in an unventilated shop with overspray almost too thick to see through. When they get sick from the fumes a lot of these people would most assuredly sue the filter manufacturer if the filter were approved for use with that contaminate.
All this however in no way disagrees with the suggestion to use a supplied air respirator which is much better than trying to filter out the contaminates. Properly used however a good mask and fresh filters are just fine for the occasional painter.