How were they supposed to know that was going to happen? You think they walked up and said, “Hi. I’m here to buy your software and hurt people with it”?
If a stranger walks up to the chef in a restaurant and offers to pay them to put some mystery stuff in the food, or someone walks up in during a surgery and asks if they can make some incisions and inject some mystery stuff, would you (as a customer of the restaurant or hospital) expect this to be allowed?
That isn’t remotely comparable. You’re asking someone to quietly alter someone else’s product, not selling the product to them. They didn’t pay him to change the extension, they bought it.
They bought the permission to make changes to customer machines that had been granted to the seller by the customer. If it's just a sale of the source code, there's no problem. But what is bought is usually the pre-existing update channel (the installed base), precisely to be able to alter the product for existing users without explicitly informing them or asking for consent.