They took due care to choose materials that would not harm the monument. Public monuments and property are there for the enjoyment and use of the public, if that usage does no harm it should be allowed. The placing of this bust pushes the limits of this usage but should still be considered fair use and free speech since the artists did no harm. Even though it will cost whatever the wages and rates for the equipment needed to move it will be, I would argue that this cost could be avoided in a freer country by sanctioning this type of protest so that the artists could come forward to remove it themselves (I'm sure they would have interest from galleries or individuals for this piece) or better yet there could be a popular vote on whether to keep it.
To me, this is akin to chaining yourself to a tree in a public park to protest its removal or to a fixture in a courthouse or city hall or placing posters or drawing with chalk in such locations. This statue is about as permanent a fixture or modification as that would be.
https://www.aclu.org/free-speech/know-your-rights-demonstrat...
