We have a hardsided suitcase that locks directly on the suitcase, not a separate little lock, like you could hook through two zippers. I know security will put on a heavy duty type twist tie if they have to search your luggage, but how do they lock a suitcase that locks directly with a key?

They don't.

You can buy a strap for luggage like that to keep it closed, but as with any unlocked luggage, someone can open it if they want to. So pack valuables, cameras, jewelry, meds in your carry-on or purse!

Some airports have security checks that will scan your luggage while you wait. Then, you can lock your suitcase once it has passed inspection.

Does anyone bother to lock there luggage anymore? You never know when it is going to be scanned because it depends not only on the airport but also the airline.
I find it easier to leave it unlock and not put any thing in my bag that I can't live without.

I don't lock my luggage anymore but I have one piece that uses a 3 digit combination lock. The combination locks are located on both sides of the luggage and while I intentionally leave it in the unlocked code, I worry that it will get locked accidently and they'll have to break the locks anyway. Any ideas?