SPF South is noted SPFs on the grade stamp. Sometimes the little "s" is in brackets like SPF(s). In the Mid-Atlantic region, I think you'll find both in lumber yards, especially in 2x4 and 2x6 in 8' and 10' lengths as well as in 92-5/8" and 104-5/8" precut lengths. The lumber buyers are just looking for the cheapest studs to compete with HD and Lowes for these particular lengths.
All of our long lengths in 2x4 and 2x6 as well as all lengths in 2x8, 2x10, and 2x12 are SPF from Canada.
Your concern is valid, too that SPFs is much inferior to SPF. Most Contractors are not concerned with (or aware of) the difference between the two.