I'm now paranoid about this. During debugging I discovered that when you do a hook_update_N() with a db_add_field() it does it in two steps. First it adds the field (making the value null) and *then* it sets the default value (0 as we have it configured). But the result is everything being null but new items could be created with 0. 

So please change to AND ws.becomeavailable IS NOT NULL AND ws.becomeavailable <> 0

I guess :-)