Here is the airline's rationale for the $75 service charge. Please don't interpret my posting as agreement with it - just an explanation.

Originally, the change fee was $25, then it was raised to $35, then $50, then $75. In addition to being a way to just generate more revenue, the high change fee is related to people using back-to-back tickets. You need to make 2 mid-week trips about 3 weeks apart. So you buy a couple of back-to backs (on different airlines to avoid potential problems). You are sure of the dates of the first trip but not the 2nd. Once the 2nd trip is set, you just pay the change fee in each direction and save a lot of money.

Obviously, the airlines don't like this (understatement?) and so they have gradually raised the change fee to discourage this type of practice, or at least to make it more expensive.