That if the festival features only one performer in the evening, and he is this performer, it's like a gig... And people who paid the ticket for that day WERE coming to see him. They couldn't hide behind the fact that it's a festival so people can see other performers and it's "not a big deal".
Like for example Lyon and Six Fours in France... Thinking about that, they shouldn't cancel these ones, since you buy tickets for the evening performance. So it's like proper gigs. There are not "real" festivals... Like Somerset House too... More "Series of gigs" than festivals...
I'm not sure about Spain, but if there are many stages and many performers in the same evening, and people pay a "day ticket" or a "festival ticket", they should cancel this one.
Just thinking out loud, but it makes sense, if i understood what THEM are doing... Even if i don't agree with what THEM are doing.