That's probably a bug when translating forms or the ranger's intent. Our rangers deeply understand the meaning of "paving paradise," hence there's limited parking at trail heads. Car details on permit forms are there so rangers can tow away parking abusers instead of cutting down trees to make a bigger parking lot.
Traditionally you'd write "N/A" or "NONE" if you're being dropped off or hiking through. I suspect that detail got lost in translation to the digital format by making all fields as required.
All that said, thanks for being thoughtful and not driving into a national/state park when it's not necessary. Rangers and us like-minded folk appreciate it.