That's probably because "il mange" (he eats) and "ils mangent" (they eat) are both perfectly correct, standard French. The case of, say "dom äter", though, is quite different. Writing "dom" for the standard "de" (even if it's more often than not pronounced "dom" in practice) is an example of setting out deliberately to indicate that you are using the spoken register (compare English "Tell 'em who you are") -- rather as if you were to write "y mange" (which is how it's frequently said) for "il mange"; I very much doubt that French duolingo would accept that!

There's a big difference between how the original "in house" courses (like French) and the incubated courses (like ours) work, so that's probably why.