Children rely on their parents for support and resources, and good behavior ensures that children get what they want. Children, however, show their real faces to their friends who offer nothing except companionship and "fun".

In the same way when a woman identifies a BB, she similarly turns into a "good girl" to win admiration and thereby resources. Being herself may get her fun, but commitment from a man is more valuable.

Part of this "win admiration" game to project herself as more noble or higher status than the BB, and shit tests get her to that point.

Promising future commitment or investment in return for sexual favors puts a girl in conflict mode, because she doesn't want to project that her sexual services are for sale. Her winning strategy is to project herself as too "noble" to sell out, because its more admirable. If she promises sex for your future commitment, the commitment will end when the sex ends, and is therefore risky. The "I'm noble" attitude has more mileage, because it requires minimal investment on her part.

TL;DR: let her know you don't intend to invest in her, she has no reason to be a good girl.