I think the weighing also helps college admission counselors. If they review a students transcript and see averages constantly around or exceeding 100 they can assume that the particular student took Honors/AP classes and can give them special consideration accordingly. Colleges these days are extremely competitive and anything to give students an edge should be encouraged. Heck, my cumulative average of 98 (I took no AP's sophomore year) couldn't get me into Columbia or NYU . Although now, i'm not complaining given tuition rates at those schools.
Side note: I was throwing in a little rebuttal to account for the many students placed into AP classes who aren't cut out for them. For them grades and college chances get worse because counselors just assume a high grade in a regular class equals capability in a more challenging class. People don't realize laziness is a huge problem among high school students (especially the ones I graduated with).