Mild spoiler warning ahead.
First McAnally's pub incoroporates certain symbols to disperse errant magical energies, such as the ones caused by distressed wizards. This is done through the number 13.
The place has 13 pillars, tables etc and probably other things. Mechanically this probably gives the place an aspect like "Unruly magic is dampened" or something of the like.
Secondly When the war between the council and the red court starts the pub was added to the list of accorded neutral territory. There's a sort of magical genevan convention in the dresdenverse. Its got no power of its own but almost all factions recognize the accords.
And in that way neutral territory means that everyone's agreed 'No fighting here'.
Nothing stops you from slinging spells there. But if you do certain people are going to get upset and anyone who's still got an axe to grind will have a legal excuse to do so. And if you're not important enough you'll probably be persecuted because everyone likes to look nice and punish the bad man violating the accords.