Great point. People often assume that if something hasn’t happened to them, it’s not possible. Immigration does its job as expected, and only a small percentage of people may be randomly questioned (whether flagged or not). It happened to me on another legit visa. If you're prepared, not a bug deal except taking a little longer than usual.
It feels like listening to the same record on repeat... People, named or unnamed, have been posting about CM immigration placing them in a separate queue for quite some time now. So what are you trying to tell us by that? That all anonymous are liars (70% of posts here)? That immigration will never be able perform checks?
The rules being applied are those related to border protection, which allow immigration officers to request any documents or ask any questions they see fit. These rules take precedence over other visa regulations. As you mentioned, there is no specific rule. Yes, no specific rule that limits the authority of immigration officers in this context.