I don't doubt you're right... I took a tax law class 10 years ago and intentionally forgot everything when it was over.
Are you saying the client resides in Massachusetts? That makes a little more sense, since he would file a state tax return and would claim your payment as a deduction on his return. But in this scenario, let's just use Dallas as an example... The employer is the team, situated in the player's state of residence. The Cowboys don't have to file a state tax return, so there's no deduction that they claim of the player's salary with the state... The team would only file a federal return.
Are you saying the Cowboys have to file state income tax returns in the other states where the team plays? Do they have to claim TV revenue, etc. for games when they play in New York? That just seems ridiculous to me.
If you don't mind my asking, what state do you live/work in? I'll go read your link though.