A few companies I have worked for have had a strict no gifts policy. Many of our customers had similar policies, to the point that we couldn't even pay for a customer's coffees if we have a meeting at a cafe.
These policies are aimed at preventing even the whiff of bribes or favoritism wrt purchasing or awarding of contracts.
Mostly this was in the Oil and Gas industry. We had a few mining clients that were less strict wrt gifts/swag. I now work in the banking industry where there are strict regulations against bribery, facilitation payments and many types of gifts.
So I find it a bit weird to hear that sellers provide "swag".