I'm not sure that going to a programming workshop for girls as a kid would have made me think what programming wasn't for girls. Coming to a "not for girls" signal involves realizing that they're having the event to increase female participation, rather than just because they want an all-girls event -- like girls scouts being just for girls.

The "for girls" labeling is also for the parents at that age. If you want to have girls signed up, you need their parents to think it's a good idea. Their parents are more likely (I'd think) to be biased that girls probably won't like programming/tech. So they wouldn't think about it as something their daughter would like, except that it's especially for girls. I'm not saying it's great logic, but it seems like it might be a benefit to "for girls" labeling on programming events for kids.