According to Mr. Douglass no other nation would have the nerve to put a law like The Fugitive Slave Law on its books. Why not?

A lot of countries have had slavery throughout history. Slavery was essentially the default labor system in most of the world from the dawn of civilization until well into the 18th century.

But the US slavery system was unusual in a number of ways, which can help explain why the US would establish such a historically unusual (as far as I can tell, unprecedented) law as the Fugitive Slave Act.

First, slavery was not legal in all the United States. Most countries had more centralized governments where slavery was either legal in the whole country or illegal in the whole country; the federalized US system was decentralized enough that there were both "free states" and "slave states" within the United States.

Second, in the US, slavery co-existed with an industrialized economy. For most of history, slavery has been used predominantly for agriculture, and is most common in countries that have heavily agriculture-dependent economies. The US economy was quite industrialized, however--especially by the standards of the time--but it was again not industrialized everywhere. Free states in the North were much more industrialized than slave states in the South. Even aside from its moral horrors, slavery just isn't an efficient system when you are industrialized. So it wasn't just moral compunction that kept slavery out of the North (many people in the North actually had no particular problem with slavery); it was also basic economic incentives.

With this in mind, we can understand the Fugitive Slave Act as a strange kind of compromise. (Indeed, it was part of the Compromise of 1850.) The South wanted slavery to be legal everywhere, while the North wanted all slaves freed. Congress was too divided between North and South for either measure to pass, so instead a compromise was struck where slavery would remain legal in the South and illegal in the North, but the "property rights" of Southern slaveowners would still be protected by requiring federal marshals to arrest and return fugitive slaves. In practice, almost no evidence was required to convict someone of being a fugitive slave, so many people who had been born free--but Black--were sent into slavery as a result of the law.

To Frederick Douglass, this law no doubt seemed extremely hypocritical (hence his line about "the nerve"), but I don't think it was uniquely hypocritical as far as national policies go, once you factor in that the US has a federalized system and states were fiercely divided on the issue of slavery. Compromise between widely divergent opponents often comes out looking like hypocrisy. Whereas most countries had a unified policy on slavery either for or against, the US was strongly divided, and so had a strange mix of pro-slavery and anti-slavery policies depending on which side won any given argument.

We might like to imagine that all reasonable people would be opposed to slavery from the start, but that simply wasn't true; millions of Americans supported slavery and voted accordingly. The real lesson here is that otherwise good people can think and do some truly terrible things--and we should perhaps think about what sort of terrible things we might be doing that we don't realize, which our descendants will some day look back on and be appalled.