Landsburg's idea on this question is fully in tune with his consequentialism:
A well-executed theft takes time and energy, which could have been used productively. If I spend an hour stealing your bicycle, we still have only one bicycle between us; if I spend an hour building (or earning) a bicycle, we have two. By diverting productive resources from useful activities, theft leaves the world an unnecessarily poorer place. ...
"Don't leave the world worse off than you found it." ...
You are productive when the benefits of actions (to everyone, including you) exceed the costs (to everyone, including you). (164-5)
Now there are other utilitarian reasons why theft it bad. First, it sows fear and panic among the populace, diminishes the security of property rights, and, if widespread and undeterred, may harm both consumption and production with severe disincentives to both.
Second, awareness of the danger of being robbed causes potential victims to spend money pro tanto on defending themselves. A pointless "unproductive" arms race is initiated between the thieves and their prey in which scarce resources are wasted.
Third, theft is a particularly inefficient form of predation. The thief may even kill a person in order to enable himself to take his stuff. Sporadic theft is an instance of a war of all against all. A more sophisticated and prudent system, for example, is serfdom, wherein the thief imposes permanent taxes on his victims. Everyone gets to live, and the victim even retains an incentive to accumulate wealth, while the thief is enjoying a permanent stream of looted income in relative safety. After all, the richer the people are, the more there is to steal. But ordinary theft just kills the goose that lays the golden eggs.
All these, however, are minor considerations.
As I have argued, utilitarianism is not an ethic. More specifically, utilitarianism is addressed not to the citizen but to the legislator. A good illustration is traffic regulations. (The government is the organization that issues them because it owns the roads.) These regulations are utilitarian in the sense that they are meant to maximize the overall efficiency of traffic, perhaps with a few other requisite goals, such as avoiding deadlock and starvation, thrown in for good measure. But an individual driver is supposed and even required simply to look after himself and seek only his own advancement on the road.
Landsburg's argument provides a reason for the government to outlaw theft, to threaten to punish potential thieves, and actually to punish thieves that are caught and convicted. This is so, again, especially if the costs of law enforcement are smaller than total expenses the citizens individually would make to protect themselves from thefts. But it provides no reason for any individual thief to become an honest man. For we may indeed imagine (falsely) the government to be in charge of "general welfare," such that its laws are utilitarian. But an individual citizen is asked not to be a utilitarian but simply to seek his own self-interest. For the thief, the benefits of stealing may outweigh the costs. The calculation proper to him as a citizen and not a legislator impels him to steal. Therefore, a different argument is needed if one wants to prove that the thief is acting immorally.
Landsburg urges us to consider our neighbor's interests on par with our own: "A cost is a cost and a benefit is a benefit, whether they're felt by you, your neighbor, or a stranger in Timbuktu." (165) But when one does that, the most he can muster is disinterested benevolence, and when animated by this type of love, one acts as a legislator. As a citizen, one can only realistically love himself and closest friends and family. This love is no longer disinterested but deeply personal. In such a case, in one's personal life, one privileges his beloved and treats all others as strangers according to mere deontological natural law.
In other words, for a large community, utilitarianism will take the form of general rules created by a prudent lawgiver. Practicing act utilitarianism can be done successfully only in the tiniest of communities, most plausibly one's own family.