Yes, I get that.
Where inventory is scarce, getting paid earlier for it is better. However, having stock sitting on the shelf and sales going to competitors rather than getting that sale and just seeing the money later is increasing your income AND reducing your expenses. There is a storage cost (FBA or not) and not getting that sale in more than 30 days, is effectively reducing the cash flow.
My argument is simply that the ability to pay Net 30 does make sales and more sales than not having it.
Of course, if the inventory is scarce, it is not as beneficial.
As mentioned by several, the number of these purchases is very small - probably less than 1% - but that could certainly be higher, especially for business buyers purchasing products that businesses normally would purchase and it is just as likely that the retailers who sell those products also see payment terms delaying their own payments.
When you consider the typical situation for most retailers, they are either paying with credit card (which gives them close to 30 days between the purchase and the payment for that item) or they can request Net payment terms from their suppliers. The financial impact nets out against the benefit, which should normally be additional sales. For a business that pays cash up front for everything, it is certainly going to be a problem, but then, they should already be looking at their own credit options with all of their suppliers.
What is the difference between making a sale today and receiving payment for it in 30 days or not seeing that item purchased for 30 days? I believe with storage costs considered, it is still a positive to have it sold.