Bombing, by some definitions, is art without permission.
Public art, by some definitions, is the town council handing over a wad of rates payers money after you've put forward a prospectus and they agreed-to or censored part to all of your vision, and green lit whatever husk survived committee, probe and commission.
Knitting takes a very long time. Even knitting the bulk of your undetonated yarn bomb elsewhere means that this "art" is going to take weeks in construction... Unless it's just something the "artist" acquired from a salvation army thrift shop, and then picked and unpicked the seams which lacks any soul to the endeavour whatsoever.