Radio signals, sure - but this is light, from a source travelling rapidly across the sky and covering a massive amount of area in it's path. And pretty much the only countries which might have the potential ability to shoot them down are also signatories to the International Outer Space Treaty they couldn't do that without some extremely serious repercussions on the global stage.

Giant advertisements placed in orbit by a private individual or group would seem to contravene several of these principles

Originally yes, in the spirit of the exploration clause it certainly would have raised issues at the time the agreement was established but as we have already seen the rapid commercialisation of space by countries across the Earth and that it is low Earth orbit there is already a precedent of at least near-space being fair game for business interests. And ironically most countries collectively already have multiple satellite constellations broadcasting to Earth albeit in the radio bands of the electromagnetic spectrum which is literally what these satellites are also doing as well, no differently - it just happens to be on the human-visual band of the spectrum. I mean, we are literally talking about light bulbs on satellites being turned on and off (an oversimplification but the point stands) which at least on paper cannot be criticized without at the same time potentially risking their own access to space for "other" purposes.