• complete control over where you live,
• complete control of your movements,
• complete control over your physical interaction with people who are not prison staff or other inmates,
• in the US almost no fourth amendment restrictions on searching your living space or possessions and so can pretty much arbitrarily conduct extensive searches for prohibited items,
that it is actually possible for a significant number of prisoners to keep their smuggled items hidden once they get them in (and that they get them in in the first place is astonishing).
It gives one something to think about the next time one is taking a plane trip or attending an event with a security checkpoint. If prisons cannot keep banned items out or from being used, how can airport security or event security do so when their powers are much more limited than the powers of prison authorities?
NOTE: the prior paragraph is not meant to suggest that we should boost the powers of airport and event security. It was meant to suggest that the situation in prisons provides good evidence that much of our non-prison security is indeed just security theater.