Prevention is NOT always better than cure - but to appreciate that you have to apply more sophisticated reasoning than an amoeba.
The same dilemma exists for screening healthy people for diseases in medicine.
'Screening' starts with the default assumption that the patient has heart disease, breast cancer whatever - and you then set out to prove the negative.
There is always a false positive error rate in whatever test you apply.
Therefore one should accept
1. normal people are going to be misdiagnosed
2. if the damage (and/or number) of misdiagnoses exceeds the benefits (and/or numbers) of correct diagnoses you abandon the screening program
Harvest (say) a million internet transactions to catch the one in a million by a p@edo/terrorist/tax dodger. Say your test has the unbelievably impressive false positive rate of 0.01 percent (1:10000) and 100 percent true positive rate.
You will 'detect' one terrorist whilst falsely accusing 100 innocent people.
Not looking so good for the snoopers - even with highly optimistic assumptions of prevalence of bad guys and performance of the screening instrument.