Video game publishers sending legal notices to streamers and such (youtube "Let's Play"s, twitch.tv channels, etc), requesting that they cease showing footage o

Video game publishers sending legal notices to streamers and such (youtube "Let's Play"s, twitch.tv channels, etc), requesting that they cease showing footage of the game has been a growing trend. Does anybody have any knowledge about the legal basis for rights holders demanding this? Or are they just throwing their weight around?