In the dry-cleaner scenario they obviously wouldn't have a great excuse for missing dime bags since they'd have custody of it.
REGARDLESS, I think the point isn't about "setting the right price" or comparing scenarios in which planting evidence is easier / harder, it's that ANY price is going to enable perverse-incentives for bad actors in at least some situations.
Same with digital files, if the same Geek Squad member keeps "finding" evidence they plant on peoples computers, the Geek Squad person is opening themselves up to a ton of risk. In order to plant the digital files you are by definition in possession of the banned files and are breaking the law. Any good law enforcement agent would realize that the same person is unlikely to find illicit files and that it is more likely that they are planting evidence.
The point is that there is not an easy payday. To get paid you must break the law by possessing the files. Plus, just copying the files to the persons computer would leave an obvious trace considering they know when the Geek Squad was there and know when the files were copied.
Even though the doctor in the article was not convicted (and I guess we'll never know the whole truth of the matter) you can bet his colleagues and friends aren't looking at him quite the same way.
