The red diamond signifies it’s a Travel Sentry lock that is security-friendly.  This means the TSA (Transportation Security Administration) has a master key to open and re-lock your bag after inspection.  That’s the key – re-lock.

All bags are screened, but sometimes a bag requires a physical search.  You would unlikely be present for this search, so your lock would be cut off.  Now your luggage has un-secured zippers, and you could be that guy whose belongings are strewn across the tarmac.

You do lock your luggage, right?  Not just to prevent it from opening in transit, but for the bad baggage handlers, smugglers, bell hops, you get it.

You’d lock your front door, no?