> However, the artists would paint over previous works, so it's not like they were permanent installations, they lasted from days to on occasion a year.
That's not true. Some of the works were permanent installations, including tribute and memorial works from artists who have died. Only some spaces were designated for temporary installation.
> There was impermanence about the art. So, i think it's mostly about not wanting to lose their "canvass" than to preserve the art.
There's no need to put "canvas" in scare quotes. There's plenty of case law that helps define what an installation is, in the context of art. This isn't a novel concept.
> Given that the canvass was a building owned by someone else, I don't see much of a case.
The owner agreed to let them use the space (and even charged them for it, along with double-dipping into tax write-offs for the building). Legally, that gives them a pretty strong claim to the space.