I have some Signature Tex 40 thread here that I used on my midarm for quilting. I was able to break the thread with the hands test. However, the quilt I used it on is fine. It has been washed only once (at the laundromat, in their largest front loader), but none of the quilting lines broke. The quilting lines were much closer together than yours, however -- maybe at most 1" apart (freehand looping).
In any case, I do think it's a good idea to take the quilt back to the store.
For my midarm quilting I have switched to Glide thread, which is polyester. I just did the break test on that thread, and it also broke when jerked between my hands. This makes me think that your Signature thread is probably okay.
How large is this quilt? I'm thinking that the most likely cause of the breakage is the washing machine (assuming it is a top-loader). If the quilt is large, the central agitator can cause the quilt to twist on itself quite a bit. This was probably more stress than the thread could take, and would explain why both the quilting lines and the binding stitches broke.