Not a lawyer but here's how it works:

Illegal usually means there's a law forbidding it.

Tickets are usually governed by the Terms and Conditions (Contract) between original purchase.

It's already possible for the terms to forbid resale.

So as it stands it's possible to sell untransferable tickets. And those who sell such tickets are in breach of contract, but are not breaking a law.

What would 'making reselling tickets illegal' entail, a law that makes selling transferable tickets illegal? That would be a very weak position. A law that upgrades the resale of untransferable tickets to a crime? Again a very weak position.

I contend that people that suggest and write laws should learn about actual law.