I wasn't question the rule... having rules is fine. The fact is that the rule's existence is dumb.
It's true that there are many silly rules in English grammar. This one in particular, however, makes perfect sense because it keeps pronunciation rules consistent (not that THOSE are always consistent, though. But anything that does actually promote consistency in language is a good thing, IMO.).
If you didn't double the consonant on one-syllable, short vowel/consonant words, you'd have to pronounce them differently. So, sad and saden (sadden) would no longer sound the same. The first would be the short a sound as usual, but the second would be a long a sound (pronounced like the first a in satan).
So, without the double consonant in sadden, we'd either have to have inconsistent pronunciation rules, or change the official pronunciation of words like that (which is really, really hard to do).