Theft from such suitcases is astonishingly easy; the locks on the zippers are irrelevant.
And, as the audio commentary indicates, sometimes the problem is not theft of items from your suitcase, but material placed surreptitiously inside in order to have you inadvertently serve as the "mule" at the border.
One way is to stabilize your lock in place. You can zip it up to the area around the handle, and then rope it, chain it, secure the lock to the handle. This way IF it were to be tempered with, then can't reseal it unless they shift the lock around. Now you have some sort of evidence of tempering.
My aunt went to Trinidad and Tobago to check out a fabric distributer. While there she was given a "gift" of two bottles of wine, which she put into her luggage before boarding a plane back to Canada. Once seated on the plane she was confronted by T&T officials who removed her from the plan stating she was smuggling two bottles of liquid cocaine. She spent two years in prison in Trinidad and Tobago for drug trafficking, but claims she had no idea the "wine" was liquid cocaine. Be careful out there.
Actually you can still lock your suitcases. It's not illegal. However, TSA does have the right to cut the locks at any time to gain access to the bag for security checks. In order to avoid having your locks destroyed, you can purchase special TSA approved locks that are able to be opened by TSA agents without being broken. You can find info about this on the TSA website and other places. I had them on my bag for a trip to Chicago from Los Angeles and agents at multiple airports commented positively about them. It seems many people dont even know they're available.
OK, but what if it's the TSA agents I'm worried about? They've been caught stealing from people's luggage.