The issue come in that Microsoft gets 30% of sales. There is a cost to doing the QA, the encoding, and then placing into the store. Microsoft wants a revenue stream to basically cover the cost of the administration costs required to place an addon in the store. Plus there is the administration cost of maintaining the hosting of the addons, etc.
In the case of libraries, they are a requirement in order for other addons in store. Microsoft has allowed those to be free considering they are getting revenue from the initial addon.
A livery isn’t required to make use of the base product. A base product will require a library though. That is why libraries can be free, but liveries can’t.