Actually... what happens is that the layer of adhesive remains as a "pad" on the envelope, and the stamp cleanly separates from the adhesive.
Good for them! It's a multi-layer adhesive. There is a water-soluble layer between the stamp paper and the actual self-adhesive layer. That's why it separates easily when you soak it.
USPS also used water-soluble layers on some issues. However, USPS no longer requires it -- so the result has been headaches for collectors of used modern stamps.
Can't say I blame USPS. The additional water soluble layer is an additional expense (yeah, they are hard up for money), and making the stamps hard to remove reduces the possibility that they can be successfully reused.