From this story in National Parks Traveler:
"...The need for the revisions arose as more and more visitors took to the roofs of their vehicles to photograph bears and, in at least two instances, the bears took exception and charged the vehicles, according to park officials. While park guidelines long have said visitors should not approach within 100 yards of bears and wolves, or within 25 yards from other animals, including nesting birds, the updated regulation now specifies that "remaining, viewing, or engaging in any activity within 100 yards of bears or wolves" is against park regulations. With highly photogenic grizzly sows No. 399 and No. 610 -- and, this year, their five cubs -- regularly frequenting the park's front country, more and more photographers realized that they could get some great shots of them if they just waited long enough, Grand Teton spokeswoman Jackie Skaggs said Thursday.
"They sit and park for 12 hours a day where they think they're going to get shots of these bears," she said. "Other people who drive by think, 'Oh, that's a great thing to do.'" The result is not just road shoulders lined with cars and trucks, but with people sitting atop those cars and trucks hoping for a great photograph to return home with, said Ms. Skaggs. When the bears arrive to this mass of humanity, problems can quickly arise, she said.