This is far from surprising. The reason for the growth in SIM-only in the handset space is that there are tens of millions of spare handsets sitting around which only require an active SIM to be placed into them to bring them back into service. Mobile broadband has no such installed base of equipment. USB modems have only been selling for a little over a year meaning there are only a few million devices in the market of which most are still under contract and few can be considered spare. Even when there are a large number of USB modems in the market, most people will opt to simply buy a cheap new modem (prices are steadily dropping) rather than get a new SIM for an old one which by then will be labouring on the previous generation of technology.
There will only be a viable market for SIM-only when large numbers of laptops are in the market with embedded MBB functionality and no ongoing contract. That won't be much before 2012.