This is my understanding after speaking to someone in AD about this when the TN Fund started. The TN fund is the donor mechanism if you want tickets as a 'reward' for the gift. This way, UT decides how the money is actually used. The value of the tickets is viewed as a 'benefit' and subtracted from your donation for tax purposes.
If you donate to a department, you do not get the ticket 'benefit' so the entire amount donated can be deducted on your taxes.
So, the TN fund is supposed to be a way to differentiate donors in terms of the ticket benefit for tax purposes but I suspect it is primarily in place so UT has the power to decide how your money is used.