The difference is "end up". They can (if appropriate, and if they can't cover it elsewhere) sell small amounts to cover the interest on the loan, rather than the large amount of the principal. At the end of the loan term the usual thing would be to roll it into another loan. The lender wants that stream of interest payments. The only time they care about the full amount is if they want (or need) to call in the loan, which, in the ideal situation from their perspective, never happens.
So the borrower should never need to sell enough of the stock to seriously affect the price, and the tax is only payable on a small number, not a big one.