Hmmm... no I have to disagree with the picture you paint here. A licence is not a 'threat to sue', it's a set of conditions for use. Theoretically you could be sued for removing the copyright notice and author attribution from BSD licenced code just as you could be sued for not making the source code available when you distribute GPL licenced code.
Furthermore when you talk about 'promising not to sue', as we see today the highest risk of being sued for software is through software patents. GPL says that if you distribute your patented code under GPL you are not allowed to turn around and sue people using that code or any derivates of it. The BSD licence makes no such provisions and as such you could release patented code under BSD and the proceed to sue people using it.
Furthermore when you talk about 'promising not to sue', as we see today the highest risk of being sued for software is through software patents. GPL says that if you distribute your patented code under GPL you are not allowed to turn around and sue people using that code or any derivates of it. The BSD licence makes no such provisions and as such you could release patented code under BSD and the proceed to sue people using it.