Legalism is all about rule-following, especially in a religious sense. The undertone of the idea is, the more rules you can follow, and the harder it is to follow them, the better "Christian" you are, and the more "religious" you are (well, I can't argue that last one, actually, but just being "religious" is never a good thing).
But think about this: Christianity as a whole, the entire concept of being a Jesus-follower, is based on the Bible.
Here's a direct quote from the Bible, from the Man we all claim to follow religiously: "You shall know the truth, and the truth shall set you free."
Now, although a lot of people (including Christians) seem to think that Christians aren't supposed to use their brains, I assure you I've thought a lot about this. I've turned that statement on its head, inside out, and run it backwards... and you know what I've figured out?
That the converse of it is also true: If a certain belief or opinion isn't setting you free, then guess what?
It ain't the truth!
Jesus wants His people to enjoy the freedoms that He died to give us: freedom from sin, freedom from bondage to anything or anyone but Him (and he has even told us that the "burden" of following Him is easy, and the "yoke" he puts on us is light), freedom from being bound by the letter of the law. He even tells us not to allow ourselves to be bound up by the laws of others (government excepted)!
So can someone tell me WHY some people consider it their bounden duty to try and make other people follow their personal convictions as if they were laws? We're not bound to follow man-made rules--we're meant to follow Jesus alone.
To do anything else is unscriptural... just as unscriptural as their trying to impose their rules on us in the first place.