I can come up with two different ways of understanding this. One's more charitable, one’s less, but neither is that great.
Here's number one: Maybe the right wing loves the 1700s because government was smaller. The point isn't that there was no civil rights law -- that’s an unfortunate side issue. The point is that there was no income tax, and America was a paradise of free enterprise. This is, unfortunately, an ass-backward misreading of history. In the early days, the big government debate worked much differently. Back then, if you wanted free market capitalism, you were for big government. Lots of people were just living off their land and not doing much buying or selling, and to drag them into the market required using state power. This was the stance of the northern, Federalist "Founders," mainly -- John Adams, Alexander Hamilton, etc. Small government was the populist stance, and was in particular a Thomas Jefferson specialty.
But here's Beck: "Do you believe that George Washington, Thomas Jefferson, Ben Franklin, Madison, Adams, do you believe those men were enlightened men? I do. Well, their crazy idea was to allow men to be free and free in their own business to allow them to be able to engage in capitalism."
So that's option one: an uninformed nostalgia for the 1790s as a mythical time when we were a nation of Ayn Rand characters, all six-foot-five, straight-backed, square-jawed, and buying and selling free of encumbrance.
This brings us to option two, however. Even if the past had been a free market paradise, it still only would've applied to the small fraction who were seen as full human beings and allowed rights as such. It's hardly a free market if you're forced to work in the fields for no pay, or forbidden from owning property. Casually dismissing these things because they get in the way of worship of the original Constitution seems revealing of something worse than being uninformed. It's almost as if the crucial rights enshrined in the Constitution only matter for white guys.
Personally, I think the fetish can be explained by the authoritarian personality meme that predominates in conservatives.