Not theory. Legality.

Cops are indeed trained to fish for information, that is part of their job. But if you evoke your right to either keep your mouth shut, or state you want a lawyer, during questioning... they cannot continue. If you aren't a suspect or person of interest in anything, they may just be asking for completeness on their end such as a witness.

You just need to know when and what, if anything, you provide and know when to keep your mouth shut. Because once you waive your right, you cannot just pull that card whenever you want in some cases.