It is stupid. Ironically, not wearing a seatbelt, has never caused an accident, and I would think the legislators would be more concerned about what actually causes accidents in the first place.
I don't know if the new law says that the lack of backseat seatbelting will allow a stop just for that, but I wouldn't doubt it.
The front seat belt law, was changed to a primary offense here a few years ago, to basically let leo sniff your car and ask questions, (the modern "broken taillight" stop), and for local communities to gain revenue. Honestly, I think the average police officer is more concerned with violent criminals and unsafe driving than seatbelts, but I don't think the IL state legislators even talk to anyone in law enforcement.
Back seat passengers in limos and taxis are apparently exempt. If this was about safety, the law wouldn't have been written that way.
More stupidity in Springfield. Instead of addressing real issues, they do this type of stuff.