It's not about trust, it's about the fact that 10% of something, or even the chance of 10% of something, is better than guaranteed 100% of nothing. You're not being taken advantage of if someone uses a BSD license the way it's supposed to be used.

If I put two choices in front of someone, the binary option of copyleft vs proprietary and they'll always go proprietary, or I give them something permissive and there's at least the chance they contribute back, the second option is strictly better. It's sort of the equivalent of having a wealth tax that raises no revenue, I'd rather have you here contribute something than move abroad and get nothing, even if the benefit is more indirect