I suspect that minors can to an extent form contracts. For example, to buy anything from a store, you have to fulfill a contract (money for product). This does not require parental intervention. They can take on a summer job. Same thing.
Parental intervention only comes when there are contractual violations (i.e. minor shoplifts, parents are on the hook for at least some if not all of the damages). Though in some cases/places, parental acceptance can be required for taking on a job as a minor.
So the argument they're making is most certainly legally sound in this aspect. The point of contention is going to be "how much can a minor contractually agree to on his/her own" and "with this much additional obligations, how can amount of obligations for each contract be verified".
The whole thing is a horrible mess because before ease of access granted by computerization and proliferation of networking of computers, most contractual obligations required some kind of face to face verification of basics. When you applied for a summer job as a teen, you got into an interview where your identity got verified.
Meanwhile today, the issue is that due to aforementioned factors, we now have countless contracts that are made without parties ever encountering each other in a face to face situation. This seems to be, on a fundamental level, an attempt to get something that is as close to "face to face" as can be generated in age of social media. I.e. other party gets to at least verify if you're specially protected kind of a human (minor), or one that is fully legally responsible for him/herself. Something that used to be done face to face for each contract that extended significant legal obligations for both parties.
But it's clumsy to the extreme, which is likely the main reason why it's being fought over. Essentially instead of "protecting the companies from having unknowingly taken on additional obligation of contracting with a minor", which the legislation purportedly is trying to do, it instead just punishes everyone who isn't a minor (both users and companies) by forcing companies to assume that unless proven otherwise, everyone has those additional obligations of contracting with a minor.