The WN log says: "The Czechia law of water conduit and canalizations (274/2001Sb) does not forbid entry to (any) sewer.(in this case were talking about underground water stream, not canalization)" Next sentences in the WN log are generally speaking about things like "Its their fault" and stuff like that.
The Motol spruit tunnels are not a canalization system, but we can liken it to a sort of water conduit. I believe it is visited regularly, i would visit it too! Visits are 100% not illegal, the place is opened for public all the time.