The same way they did before 9/11™. Which some places worked quite well, others not so much.

Bad example; ABC Security got its noncompetitive contract at OAK through a corrupt relationship between Alameda County officials and the cousin who owned ABC. ABC's lapses went unpunished due to responsibility shared between Port of Oakland and the various airlines assigned to different gate areas—no single entity had sufficient control of ABC to enforce contractual obligations.

PoO made other questionable decisions as well, such as a hiring program for drivers of off-airport rental car company vans with an offender-rehab organization (!). But if the offender van drivers as a group had consistently perpetrated as many thefts or other abuses as TSA has since its inception, the provider could have lost its contract and been replaced.

Both of these issues would have been correctable by better contract vetting. OAK competes with other Bay Area airports for revenue, so market forces eventually would weigh in on repetitive problems. Security (theater) under TSA, centrally controlled with no stake in efficient function of the facilities it "serves," cannot reach even this defective level of accountability.