This rule increases the efficiency of the market. There are two types of people who pay extra for assigned seating:
1) people who need to sit together because they are flying with someone who cannot sit alone. If the fee makes the flight too expensive, they'd rather not fly or will choose a different airline without the fee.
2) people who will pay money to avoid a bit of inconvenience. If the fee makes the flight too expensive, they'll just not pay the fee but will still pay the base fare.
Making the people in #1 pay the fee loses airlines money. Making the people in #2 pay the fee is highly lucrative.
This rule will allow the airlines to make more money because they can now easily target #2 without targetting the people in category #1. Before the law this was more difficult -- how do you properly distinguish between people who have to sit together and people who want to sit together?