Would liberals please explain why union workplaces are exempt from SeaTac’s $15 minimum wage law?

The Washington Post recently published this article on the new $15 minimum wage law in SeaTac, Washington. The article states:

“… union workplaces are exempt…”

This exemption is wrong because the law is supposed to treat everyone equally.

But also, it is an extreme example of hypocrisy. It was unions who got the ball rolling on this $15 minimum wage. It was unions who said that all workers should be paid at least $15 an hour.

But now, these very same unions are getting an exemption from the very same law that they so strongly supported.

Would liberals please explain why union workshops got an exemption from SeaTac’s $15 minimum wage law that was initiated and supported by these very same unions?