Is that actually true, absent a contract or agreement that you get the copyright along with the commission? I would think the artist gets it by "default" unless you're also paying for the rights or something?
If an employee makes something at work, the employer is legally the creator of it.
If someone contracts someone else to make something for him, the copyright in the work vests in the hiring party essentially when it's part of a larger work and the contract says it's a work for hire. A one-off commission on its own does vest in the artist.
(Disclaimer: I'm a lawyer, but I'm not your lawyer.)
[1] https://copyright.gov/circs/circ30.pdf [2] https://en.wikipedia.org/wiki/Community_for_Creative_Non-Vio...