Because BB is closing the action and getting a good price, and this makes it profitable to defend more hands. Of course the latter is assuming a normal opening size and no 3-bet. It also matter, if there is an ante, how large the open raise is, and from what position. Against a min-raise from BTN in a game with antes, BB can defend any two cards or very close to it, especially if there are no ICM implications. However against a 3BB open from UTG in a game with no antes, BB should defend much less. And if someone is making an oversized open like 5-6BB, then the price matter less than position, and BTN might actually have a wider range for getting involved than BB.