Six of Crows- Chapter 30 (Leigh Bardugo)
Perhaps because a thief ~chooses~ to do (material) harm to others, while Grisha are punished for "the crime" of existing?!
It's bizarre, how Jesper IS Grisha, but he's simply completely disregarding what's happening around him, unless it's literally in front of him in the most explicit way.
Sure, he might have other problems, but this is a beautiful example of how being Grisha and understanding what it means are two diametrically different matters.
And no, not being taught about it is no excuse. He's old enough to live by himself, gamble, attend and quit "adult" school, he's old enough to know about AND understand common state of matters. It's not like he's hiding his abilities only to avoid Nina's recruitment. He lives in fucking Kerch, and voices what's happening to Grisha there.