It doesn't make sense to me. My husband is an attorney, and there are many courthouses (even county ones) which require id checks, scans of briefcases and bags, etc. Why should the Capitol have less security? Most of the guards probably know the attorneys who regularly practice in the court... but they still do the checks.
You are correct. Everyone should have to go through the metal detectors, including congressmen/women. How easy it would be for someone to counterfeit a pin and get waved on, bypassing security.