PETA is a business, and like any business they must be able to sell goods/services in order to bring in revenue and stay viable. PETA is classified as a non-profit organization, but the rules still apply.
Business ---uses marketing to ----sell services/goods = Profit
PETA-------uses marketing to---- sell services/goods = Tax Free Profit & Donations
PETA ------uses shock value videos/advertising to-----sell merchandise & warm fuzzies to the public = Donations & Profits
PETA employs a sales force to promote their videos, one of them came here: Colin Henstock. Rather than call them sales people, they label them as "Investigations Specialist". Notice how he was willing to push the video, but the moment he was asked a question he disappeared.
PETA, like any smart non-profit, will maintain a database of all previous donors. These donors will need to be approached at least yearly if not more frequently in order to bring in continual donations. Since PETA essentially is in the business of selling "warm fuzzies", they need to produce continued/new "shocking" videos to tug at the heart strings of their donors. PETA knows that the more shocking the video, the better the donations are for them. They have zero incentive to find anything except "shocking" during their hidden camera extravaganzas.
Approaching donors over the same issue, over the same company, over the same kind of animals would grow old real quick. How many times am I willing to donate to save minks? But if I'm approached with a fresh new target, say chickens, suddenly my donation wallet is opening up again.
Why target RBM? It was only a matter of time before they targeted a "Herp" company. I would suppose because of it's size and the ability to hide their "observers", RBM simply appears to have drawn the short straw.