In the US at least, any work you do for an employer and get paid for is covered under what are called work-for-hire laws which assign the copyright to the company, unless you have a written contract stating otherwise. This is true regardless of what equipment you use, and there’s absolutely zero legal barrier to a company asking you to use your own equipment in the course of a job without any compensation. The fact that your ultimatum wasn’t met with a “lol no” from Legal is pure luck.