Is this legal? A conference organizer wants my w-9 as proof of employment

After registering for a conference in Baltimore MD, the organizer wants to see a copy of my w-9 or paystub to show proof of employment in order to attend the show. I do contract work for a local company in Maine and filled out a w-9 for that local company, since I'll be paid for work performed.

 

But I am not seeking employment with the conference organizer, nor expect to be paid in services or products by the conference organizer. I just wanted to attend the trade show conference along with a representative of the local company I do work for. Is demanding a w-9 or paystub in order to attend a conference legal? If not, to whom should this digression be reported?