Because if the leg is detachable, that defeats the purpose of attaching a monitor to it. Unless your goal is to keep track of the leg, in which case no problem, but normally that's not what you're after.
In August, The Guardian reported that two employees of a private security firm had been fired for failing to follow procedures when attaching a monitor to a man under a curfew imposed for drugs, driving and weapons charges. The firm, G4S, has a contract with the UK's Ministry of Justice to do such "tagging" and reportedly tags a remarkable 2,000 offenders every week. "Given the critical nature of this service," said a G4S spokeswoman, "we have very strict procedures in place which all of our staff must follow. In this individual's case two employees failed to adhere to the correct procedures when installing the tag. Had they done so, they would have identified his prosthetic leg." Because they did not, he was able to go out whenever he liked while his leg stayed under house arrest. (I'm guessing he had a spare, like this guy did, but it isn't clear.) He had fooled the employees by wrapping his leg in a bandage when they came by to set up the monitoring device.
By saying that the employees failed to follow procedure, G4S obviously is putting the blame on them, but I'd like to know exactly how its procedures deal with this. Is there in fact a step in the taggers' checklist that says something like, "Before attaching the tag to the offender's limb, ensure that said limb is in fact attached to the offender's body"? You'd like to think that you wouldn't need to spell that out, but apparently you do. (It has happened before, or at least I infer that from the ministry's statement that "incidents like this are very rare.") One report says that the employees "failed to carry out the proper tests," and if there are particular "tests" that go along with this part of the checklist ("Step 7a: yank on leg") I'm even more interested in finding out what those are.
The matter came to light when the man was arrested for driving without a license, without insurance, and possibly without a leg at a time that he was supposed to be under curfew. The reports claim that G4S managers had "become suspicious of the situation" prior to that time, but don't say why. I'm hoping their computers were telling them that the subject had been propped up in the closet for weeks on end, or something like that, but that is just one of several unanswered questions about this case.