Translation:The boy drives straight.
"The boy rides straight THROUGH" was marked incorrect.
From my understanding "rechtDOOR" carries the sense of "through" something.
Translating in reverse, "The boy drives straight" would become "De jongen rijdt rechtUIT".
What am I missing?
What you are missing is that you are making the wrong assumptions. Rechtdoor does not mean in that you drive through something.
"Hij rijdt rechtdoor de groep mensen" would be incorrect?
It would be. You could use Hij rijdt recht door de groep mensen
In conclusion, rechtdoor (straight ahead) and recht_door (right through) are not the same thing.
Door does not only mean "through", it also means "on" or "onward", e.g. het gaat door, "it goes on"
Why is "straightforward" incorrect?
In answer to the above, straightforward as one word means honest.