Here is exactly why:
We didn't use Surepost for years. We offered either USPS Priority or UPS Ground, depending on weight and destination. In a nutshell: Ground is cheap and really fast close by (shipping from us to, say, Charlotte, NC) and Priority is faster across the country. However, Priority gets very expensive with high weight shipments.
UPS changed their rules about shipping tobacco to require that all UPS shipments get Adult Signature upon delivery. This is both expensive (about $5) and a pain in the ass for customers (and we're age verifying at checkout anyway and therefore compliant with federal law).
We stopped shipping UPS entirely to consumers while we sorted it out with UPS.
It became apparent that Surepost could go without Adult Signature and Surepost is priced (in alignment with, though less) than Ground.
Hence, we started shipping Surepost because continuing to ship 4lb packages that we sell for $100 by Priority was untenable.
However, now, in round 2 of implementation of that (that was deployed last week; I don't know when your order was, BROBS), we're subsidizing non-free options. So the difference between Priority (for which you would pay something) and Surepost shouldn't be all that great (to figure it out properly, I'd need the weight, value and destination zip code of your package).
So, why did we do this? Because UPS pulled our preferred (more expensive to us) service and USPS Priority is just very expensive on a per pound basis after the first couple of pounds, so we need some way to ship 5lbs of Lane 1Q (or a bunch of tins) cheaply enough that we can offer free shipping at $95.
Sykes