walls are public spaces in the same way as the visual field invaded by advertising is. there's a version of this same problem with radio in the idea (now more or less purely an idea) that the airwaves are public. anyway, there's a great short film you might (or might not) enjoy:
http://www.imdb.com/title/tt0303289/
i think there's something different going on with detournement than not wanting the adverts to be present--detournement is a political action that is explicit, public and ephemeral. removing them is replacing one overall aesthetic regime with another: either way, it is a decision made by authorities, where detournement is a creative (or destructive) political act made in (sometimes) an artistic manner. i like that--small gestures of defiance that are translated into a considered gesture in a public space.
like i said, i think this action is most powerful in the doing.
another way of looking at this is to wonder who decided that art has to be confined to galleries and at what point the decision (was there one?) taken that art=commodity?
why does that even make sense?