Because influence being illegal means "speech which causes an impact" is illegal. You don't want a law that outlaws propaganda because whomever decides what is propaganda has massive power.

There isn't such a thing as a non-manipulatible feed. Even a chronological feed can be manipulated by spam, so it is a bit like asking why is it okay for tools to be made of mass when it means they can be used to bludgeon someone to death.