I imagine the ToS were written as such in order to make sure some features can be monetized (like offline or screen-off playback). But this just means they would not offer any feature for free that undermines this decision. A "0px" video would effectively turn it into an audio-only stream and be an "official" way for others to bypass some of those restrictions. It would make streaming to all devices that now require a Premium/Music membership, or stream ripping (something they don't actually sue for but insist it's illegal) fair game.