In most cases, it's so they won't be liable under warranty for shoddy work. Now, you know how there's a thing in your device that changes color to indicate water damage? Maybe they could just say "breaking the seal voids the warranty, you're on your own now, sucker!" The problem isn't the "right" to repair your own stuff. The problem is people who expect to have it both ways - when they screw up their own repairs, break thing even worse, and expect the company to pony up under a warranty claim. It's THOSE people you should rant against, not the manufacturers.