The law that those three JP students dug up that everybody but longtime JP resident Michael Reiskind wanted to forget about was passed by the legislature in 1993. You can read the entire law, "An Act Furthering The Establishment Of Multipurpose Arena And Transportation Center" (in which the state also agreed to give the MBTA the land it would need for the new North Station megastation), but here's the relevant section:
SECTION 7. In consideration of the property interests and easements authorized to be transferred by this act, the new Boston Garden Corporation shall administer, produce, promote and sponsor no less than three charitable events per year at the New Boston Garden while the new Boston Garden is in operation which events shall be in consultation with the metropolitan district commission, and shall pay the net proceeds, after the deduction of expenses of said events, which expenses shall not include any rental payment for the use of the new Boston Garden, to said metropolitan district commission.
Said proceeds shall be used for the construction, renovation, modernization and rehabilitation of facilities and land of the metropolitan district commission; provided, however, that said proceeds shall not provide for compensation of employees, including independent contractors and consultants other than those deemed necessary to meet the purposes of this section; provided, further, that said proceeds shall not be used for any purpose other than described herein.
H/t John Keith for rummaging around the state archives.