That is not going to happen any time soon, but in theory they can classify anything as illegal, walking on street, using paper money, owning gold, owning bitcoin or any other property.... oh wait some of those things were already illegal once upon a time.
One more reason why Lightning Network won't be made illegal is because they prefer to track in and get more tax extortion from people, hence why Chainalysis recently announced they are integrating and tracking Lightning Network for regulation compliance.
It's a well know fact that Chainalysis is working with many exchanges, companies and government agencies.
It’s probably because the government doesn’t have the regulatory-framework to work on to start shutting down Lightning routing nodes. They can’t stop centralized shitcoins, how can they censor Bitcoin and any decentralized offchain layers build on top of it.
Plus Chain Anals’ tracking of Lightning transactions actually helps increase the liquidity in the network and helps it scale, and makes it function better.