actually they mention in the first couple episodes that hosts cannot see things that will make them question their reality
Technically they can see them. they aren't supposed to process the images... so they ignore them.
The same way you don't process tasting your teeth or gums. It isn't that that they don't taste like anything, your brain simply continually ignores the taste.
This is why certain robots like the original model of Dolores' pops malfunction. Cause for some reason they do try and process the images they are seeing.
If you did register the taste of your teeth and gums and the roof of your mouth you would go through sensory overload. So your brain is protecting you.
Weirdly enough, they would actually have to process the image in order to know that its a banned image and omit it from registering in the first place... so all of the hosts have seen and processed the banned images, they just dont have access to reason what they are... or they dismiss them as dreams.
but you guys seem really into it so I'm gonna start tonight. Yall better be right 
