San Francisco has become the first city in the nation to outlaw chewing tobacco from its playing fields, including AT&T Park, home to the San Francisco Giants. Mayor Ed Lee on Friday signed into law an ordinance to prohibit the use of smokeless tobacco at athletic venues. The new law adds smokeless tobacco - moist inhalable snuff and chewing tobacco - to the list. Minor league baseball has banned chewing tobacco since the early 1990s, but it's still a negotiable contract item with Major League Baseball players.