Using a false category of report is considered false reporting
If someone is AFK, report them for AFK.
If you choose to report them for verbal abuse instead even though they didn't say anything in chat because you think verbal abuse gets them punished faster, it's not right. Just report them for AFK. You can question the system and wonder why the AFK report takes a longer time to get a punishment, but you shouldn't change the category of the report. It's not your call and while you may disagree with Riot (I think AFKs aren't dealt with quickly enough as well), just report them for AFK and post on the forums asking for more action against intentionally AFK players.