Scenario one: you borrow some money. You exchange the money for shares. The shares go up, you exchange the shares for more money than originally, and so pay back the loan and you have a profit. If you get it wrong, they go down and you lose instead.
Scenario two: you borrow some shares. You exchange the shares for money. The shares go down, you exchange the money for more shares than originally, and so pay back the loan and you have a profit. If you get it wrong, they go up and you lose instead.
Symmetrical, yes? Very pleasing.
Indulge in #1, and no-one cares. Indulge in #2, and everyone hates you. And bans it.