Regardless of the assumption of how many people are nefarious or how actually effective TSA is, your point stands.
If they screen normally 99% of the time, then the occasional relaxing of the checks just means that the effectiveness has gone from X to 0.99*X.
TSA is security theater, but this isn’t the evidence of that.
Also, if ICE was only going after the “real” criminals, what the fuck do we need ICE for? We’ve already got a stupidly-overfunded law enforcement and prison apparatus.
The fact that we need a separate agency to go after immigrants pretty much admits that the intent is to go after non-criminals.