You should do some reading on the birth of ecommerce. This was a very explicit tradeoff that these businesses decided to make. That's why free returns are pervasive.

Now whether it's a good thing wrt the environment etc...

The return policies are indeed meant to be used the way you find problematic. You can buy a computer to use while yours is being repaired. Then return it. Employees will instruct you to do this.

You are mixing up "being an asshole" with "doing abuse". Ordering 10 t-shirts and returning 9 of them could be classified as assholery (subjectively, I'd classify it as that), while it's very likely not abuse, we'd have to check the ToS of a specific seller.

Buying 3 different sizes of a shirt because manufacturers refuse to standardize on sizing information and returning 2/3 of them seems like a standard practice for buying clothes online. Not sure why you think the stores don't account for the returns.

It's also abusive for Amazon to delay your order for a week with their Super Spiteful Shipping, pushing you to sign up for their sunk cost fallacy subscription. When I'm in the middle of some project, I've got no qualms about ordering 2 or 3 different types of some part I need to hedge. I've been burned too many times with ordering only one type, having it have a problem, and then having to wait again. Constant games with prices is another one that induces orders from me that are destined to be returned. When a price drops significantly, the times I've asked customer service about having the price adjusted they tell me to just return and rebuy (hoping I'll be too lazy), so that's what I do. It's abuse all around, I'm not going to put moral weight on my own shoulders while the amoral company has no such burden.