I don't think they want to enact an absolute ban on photography, given that they cater to tourists. People will want to take pics of the entrance, decorations, boats, and each other. That's the sort of happy activity Disney will want to continue to encourage.
They'll keep an eye out, however, for someone who appears to be taking pics of things that wouldn't normally interest your average tourist. Security set up, general layout, etc.
And if they see someone taking pics that aren't the usual touristy ones, they'll approach them in a polite and friendly manner to assess them one-on-one. Do they seem nervous? Are they behaving oddly? You can tell a lot about people, just by chatting casually with them.
Disney security is trying to walk a fine line here. They want to keep people safe without making them feel unduly harassed or scrutinized. Security has to make a judgement call. Someone who is relaxed, happy and has a reasonable explanation for the pictures they're taking shouldn't necessarily be told to stop and surrender his camera for an inspection.
Yes, they might be lying, but chances are they aren't. And there's no reason to make an innocent guest feel like they're a criminal for taking pictures while on vacation.