You are quite right, to an American, this is bizarro. Abuse of police power happens here so often, with public officials far exceeding their powers on paper, that we're also a might more paranoid when government puts it in writing with the expectation that they don't really mean it (except for those "other people"). Further, we have hordes of activist attorneys looking for just such an opportunity. Such a law would either be struck down by some activist judge before you could sneeze or would be enforced on everybody. Laws assuming selective enforcement just don't happen on the criminal side. Civil cases are another matter.
We have a more convention-based legal mentality - police or other government power are often limited by how it is traditionally exercised as well as what the laws related to that agency have granted (note the convention is unwritten but observed - any serious break with this convention is discouraged in courts). But the United States often doesn't have this convention - it is in fact run a lot more like civil law countries with regards to how much power government actually has with respect to what the law has granted.
It is no wonder this slippery slope thinking is much more prevalent in America than the Commonwealth or Britain.