People seem to forget that by selling material to the SB subscription model they don't only compete with specific clips of themselves. They also do this for every other contributor that has similar clips in this category.

I'm sure SB are not randomly buying footage, they try to offer a membership pool that is as diverse as possible.

So maybe selling a small amount of clips for a one time fee doesn't seem threatening to you, but imagine if everyone is doing the same thing. It means they can soon offer usable clips in almost all categories, making all of your portfolios much less valuable on other stock sites.