The law

So, I'm watching the Daily Show (available streaming online, sooooo awesome although I do wonder why on earth Viacom allows it to happen since they can't make any money from it), and there's a big section about the stimulus package and how the Senate's debating it. Which led me to an interesting question - I've always wondered why it's possible and even common for a bill to have a seemingly unrelated rider, and how that comes to be. Although I don't live in the U.S. anymore, I try to somewhat keep up with the news, and as I recall the stimulus package was turned from a 3- or 4-page document from the Fed to a bloated legal behemoth. And now, of course, the thing's got to get debated, but I'm getting distracted - back to the original question:
How are these riders getting through? It seems that both parties in both houses are guilty of this - how do they get away with it? For example, check this story for a Democratic abuse, or just look at how the Real ID act came into existence (although there are maaaany examples to choose from, these just popped up first in Google for me). I also found this gem of an answer where an Irishman asks this question in plain and simple terms, the response to which is a (admittedly pretty good) summary of the two legislative houses and their functions and interactions. Later on, as an update, someone points out "nice explanation, but you didn't answer the question" and the response is "It happens, deal with it". Nice. Thanks for making me read your summary of the legislative branch, asshat.
Anyway, any legal types or those who hang out with legal types or especially those with hard-working friends in Washington, please feel free to let me know how things like the Real ID act (or, if you don't want to pick on Republicans, use this as an example) can make it into seemingly unrelated bills. I don't recall this part of the process being described when I received my early education about law...