Christ on a pogo stick you honkeys are funny
Am I the only Middle Easterner here? Do I have to bust out the brown-people mojo?
Pointing your feet or throwing your shoe at someone is considered rude because it is considered rude. There is no super secret symbolism ("I walk on shit, therefore you are shit!" or "I stand on the shoe and the shoe hits you therefore by the transitive law I stand on you!") If there was a super secret symbolism, it has been lost in the mists of time.
Basically, the analogue is like chewing with your mouth open. It is rude and disrespectful. No one goes around trying to trace the lineage of why it is disrespectful ("I chew this food, and by showing you the food, I symbolically chew on your hopes and dreams! Gnash gnash!") It's just something you Don't Do.
Please stop trying to make all us camel jockeys look like those aliens from Star Trek: TNG who could only communicate in metaphor. While Darmok and Gilaad at Tanagra is great and all, sometimes something is rude just because.
Here: you want a reason? I'll give a reason. Throwing anything at anyone, unsolicited, is rude because it is a physical threat. And shoes are generally nastier than, say, bouquets. Boom. There's your fuckin' symbolism.