Not following what part is illegal... brewing is legal, most places the host can provide free beer(if the venue allows it)...
Homebrewing is definitely legal. Selling it...not so much. Nor is providing it at a venue that has a liquor license. I WILL be bringing my own brew to my own daughter's wedding next month, but only to the rehearsal and with a picnic tap. I can borrow one of my buddy's kegs if I need to (licensed brewery) but only if I can prove the beer was brewed in a licensed brewery. Also crossing state lines (WA to ID). I don't want to take the risk. Still want to get the stupid neighbor for something though. I've called WA state HUD but didn't get a response. Imagine that!