Should firearms for kids be illegal? Should driving for kids be illegal? Should gambling for kids be illegal? Should starring in porn for kids be illegal? Should data-mining the behaviour of children be illegal? Should paying children to work in mines be illegal?
Obviously the principle would be something along the lines of "games with microtransactions are potentially hazardous to the buyer in a way, or to a degree, such that while we should allow adults to make their own choices, we should protect children from the consequences of being partially-formed minds, and so we should avoid targeting children with that business model". Same as gambling, or selling your PII to get free services, or starring in porn. I'm not saying it's a slam-dunk argument, I'm saying the structure of the argument is obvious and straightforward. And I expect every adult to be at least partially sympathetic to it, even the ones who end the paragraph with "but, on the balance, freedom is more important, so it should not be illegal".
Well. I admit. I mostly agree with GP's claim. But not with high confidence.