My best guess: mods don't personally review all the memes. With the amount of memes submitted each day, they'd be here all day doing that.
I think the mods only look at memes on originally uploaded templates, and memes that are auto-flagged for troublesome words, and maybe a random sampling of others here and there.
I think if a meme is on a site standard template, they just let imgflip programs decide whether to feature it or not - and it's probably set up so that if it has one English word or word fragment anywhere in the meme, the programs give it the benefit of the doubt and let it pass through.
That's my best guess.