But they aren't buying a product, they are providing one-- the output of their labor.
That depends on whose point of view you're looking at. From the employee's point of view, yes. From the employer's point of view, they are the one paying money for something, which makes them the consumer. Therefore, labor (or you could argue skills and time) is the product employees are selling.
If one feels that their labor is worth more than what their employer is paying, they are free to move within the market and see if they can find another purchaser of their labor at their preferred price.
And if the purchase of their labor can be arranged through a third party, is that third party not also a market force?
all the union does is distort the price for labor
If indeed the union is a market force, then its price for labor is not a distortion--unless the union is the only market force on that side of the equation, i.e., a monopoly. If it is not a monopoly, it is simply another competing provider of the product. The union can charge more for their product because it has been upgraded (much like a staffing company) by a guarantee of a certain skillset.
Now take all of the above with one caveat: I'm talking about laboratory conditions. In the real world, unions get monopolies, they're corrupt, they protect the lazy and unskilled, distort the job market, yada yada yada. But employers aren't any better. Before unions, they abused and distorted the market. Child labor laws, the 40 hour work week, minimum wage, and safety standards all come from union efforts. Until unions began asserting some power in the marketplace, employers were the monopoly, and the market value of labor was low wages for long days and shitty working conditions. Even with all the progress made, to this day employers will still find a way to screw you over if it's good for the bottom line and legal, or if not legal, the penalties cheap enough.