PETA seems to be more focused on mammalian rights but anyone who crusades for the protection of silkworms would probably also take up the case of flies.

That's probably why there is the comment at the end: "No fly was harmed. The banners were attached with wax and detached themselves after a couple of hours.". It's up to you, whether you want to believe this... :-)

They tend to be pretty trollish about it. They'll probably complain about the flies being "tortured" by having to fly with a weight attached to them.

At the end it claims that no flies were hurt and that the ads were attached with wax, which came off on its own after a few hours...