Well, the argument that is used, and it makes sense if the deal is fair to both sides, is the sports team brings millions in new tax revenue through commerce.
Historically, governments have been hoodwinked into these deals with lofty projections that dont pan out. San Diego made a deal with the padres based on ticket sales that totally backfired on the city. I think bottom line is folks in government are not savvy enough to strike a good deal and the capitalist pigs savage the taxpayer.