Power outlets are freely available for use in public libraries. I charge my laptop and cellphone when I'm researching there, its there for public usage. It is definitely not considered theft and is the norm. Charging 18650s in a laptop, charging 18650s in an ebike pack.... hmmm. Intention of use is different, but again, its public and freely available. I don't consider/ see how it can be theft.
Jumping over a fence to a publicly funded school, going into a classroom and charging without asking? Yeah... no thanks. Going way too far for my liking, but nothing to do with theft, everything to do with trespassing and asking for trouble.
I don't its a cut and dried issue, I think it depends on context, notification available and a very good dose of common sense.