Totally incorrect.
The restrictions you're thinking of were (for the most part) lifted in 1996, and any software S/MIME implementation would almost certainly be exempt.
Plus, if we were still living in the pre-13026 days the iPhone would ship with a reduced key-length SSL implementation (which, to the best of my knowledge, it doesn't)...
No, the real reason that Apple doesn't support S/MIME is that the average consumer doesn't know or care about it and (despite PR campaigns to the contrary) they're not really aiming the iPhone at business users. That's fine, as it allows them to make what is arguably the best consumer phone out there, but it does mean that folks that want IMAP IDLE push e-mail, S/MIME support, CalDAV calendar sharing, BIS integration etc. will have to look elsewhere.