Because it's comparable to the value of their 1st pick points wise. So even though they lose their 2018 1st, they get back our 2018 2nd which, when combined with their own 2018 2nd, can be upgraded to a 1st rounder under the F/S points system. They also get our pick 30 this year to make up having to use their 2nd rounder next year for West.
Basically, it evens out to: we lose 11 + 30 (2nd rounder), Dogs lose Stringer + 2018 2nd rounder.