Illegal for pros

I was reading an old issue of Zymurgy that had a recipe for Charlie P’s Bourbon Barrel Runnings Porter.

He adds the cleared sludge from a barrel to a porter. In the text he explicitly states that it is illegal for commercial brewers.

Any idea why? Just curious.

You are not allowed to “fortify” the beer with distilled alcohol.

OK, if the beer extracts it from the barrel naturally, it’s legal.