Fred Phelps, inadmissible
The exception would be if there's a good reason to think that the offense in question is more likely to occur in one country than another. Thus, if a Yemeni al-Qaida sympathizer is arrested for planning a terrorist attack in the U.S., it would make sense to deport them, since al-Qaida's ideology does not hold that Yemen is the Great Satan and ought to be attacked, so the deportation will lead to a net decrease in terrorism.
So I was given pause by the news that Britain has banned Fred Phelps and Shirley Phelps-Roper -- of "God hates fags" fame -- from entering the country.
Melissa McEwan says the decision "recognizes the fundamental difference between speech and incitement to hatred." That's part of it -- if there were no incitement to hatred, there would be no grounds for exclusion*. But while incitement is a necessary condition, it's not a sufficient one. If the exclusion were simply on the basis of the Phelps' general propensity to incite hatred, then excluding them just shifts it around. If, say, the Phelps were just coming for a vacation to see Big Ben and Westminster Abbey, then barring them from entering Britain protects British LGBT folks at the expense of those in the Phelps' alternative vacation destination -- say New York City, which doesn't have the power to ban American visitors.
But the Phelps aren't coming to Britain for a vacation incidental to their hate mission. They're coming in order to organize protests against The Laramie Project, a pro-gay play at Queen Mary's College. Presumably nobody in the U.S. is putting on The Laramie Project or any other play comparably offensive to the Phelps -- otherwise they wouldn't have tried to travel to Britain to do their protesting. So in this case, excluding them will result in a net decrease in hatred incited, and thus is justified.
*I generally take a more expansive view of what things should be counted as free speech than most people who otherwise share my political opinions, but I agree that the Phelps frequently cross that line.