It's nothing specific to Kandel, I just find the standards of scholarship practiced by textbooks to be poor. Like I said, the book will be filled with claims like:
"The optic nerve is directly connected to the superior colliculus"
It seems very factual and set in stone but I bet if you read the primary literature there will be variation and doubt. If you read my last ref you will see they claim direct connections between CST and motorneurons in rats of some ages but not others. Perhaps this optic nerve claim was made based on using animals of a certain age, so it won't generalize. Who knows? That's why there should be a citation.
tl;dr Current textbook practices promote false certainty, and I don't think it is helpful for learning about a topic.
If you really have a problem with Kandel, use email. Most authors of these types of book NEVER get any email about them and would be thrilled to have some interaction with a reader.