In principle, it's probably not that unreasonable. You're not going to get arrested for running onto the stage at a gig or a play, just chucked out by security and banned from the venue. But if you lamp somebody in the process, you're going to get arrested.
That said, although it's not specifically an offence in Scotland, I'm surprised a pitch incursion can't just be considered a breach of the peace, or some other nebulous public order charge.