If I have a client which needs to run something requiring a license, they purchase a license (through me) so they can continue to use the licensed product.
If I purchase a license, it's so I [my company] can legally use the product. So I would be free to show mockups or other things using the product. Then if a client chooses a mockup containing a licensed product, the client pays for a license so they can use the product.
It's not only ethical, it's how it should be done. If you fail to purchase a license for your client, then you need to relinquish your license to the product. If you have only one license, in most instances, only one company/individual can use the product.