Since shoplifters use roads to make their getaway, SOPA would have us set up roadblocks to catch all the thieves.
Could we catch more thieves by setting up roadblocks? Of course! So why don't we? For the same reasons SOPA would be bad law: we don't stop and search people in roadblocks (4th amendment prevents unreasonalbe search and seizure). Also, as a practical matter, roads, like DNS, are shared resources and the collateral damage to transportation (and internet routing) would be massive.
Using an analogy to aid understanding can seriously backfire when your opponents make fallacious arguments from analogy, because the audience doesn't actually understand the issue so they go with whatever the analogies makes believable.