According to the article the building owner allowed the "artists" to use the building to do their "art", so they did nothing illegal. However it is silly to think that somehow they were going to retrieve their work or that a museum would want it.
$1:
The creation of 5Pointz dates to 1993, when Wolkoff first let visual arts use the derelict Long Island City property he owned as their large-scale canvases.