It's not ridiculous. The standard advice to anyone who hires a contractor is to get a W-9 before any work begins.
My advice to you is to give them the W-9. If I were the law firm I would insist on getting the W-9 before you started any work, and I certainly would not pay you anything until I got a W-9 from you. They don't want to worry about having a hard time getting a W-9 from you if you should happen to do more work for them and they end up paying you $600 or more for the year. Contractors are not always cooperative about providing a W-9 after they have been paid.
