Is this something that's somehow mandatory by state law? I'm just trying to figure out why a coach of Kelly's caliber would've ever agreed to this in his contract
Its kind of standard, really, especially when the buyout is 100% of your remaining salary. Jimbo Fisher and Billy Napier did not have offsets from future employment, but they only received about 80% of their remaining salary.
The buyout clause is really a liquidated damages clause to avoid litigation. The parties agree that if LSU terminates the contract early, then Brian Kelly is due monetary compensation to make him whole. In this case they agreed he could keep drawing his remaining 6 years of salary.
But, generally, everyone has a legal duty to mitigate their own damages, that is, you take reasonable steps to reduce your own financial loss. If a contractor damages your roof, you don't let it rain in your house and expect the contractor to pay for all the damage...you put up a tarp or something.
In employment law, if someone is wrongfully terminated they cant just sit at home and wait for their fat pay check to keep coming. They have to take reasonable steps to find comparable employment and mitigate their own losses. If the person does not take another job, their judgment might be reduced by the amount they could have earned.
That's what is happening here...LSU says it will continue to pay the agreed upon salary, but it will be offset by whatever Kelly earns elsewhere. And Kelly has to make a good faith effort to find something.
James Franklin had similar terms. When PSU fired him, he was due $49 MM over 7 years. When Franklin was about to take the Virginia Tech job, he accepted a $9 MM lump sum from PSU.