It doesn't allow tethering, unlike The One plan.
I believe it will in practice, though what you say is Three's official position admittedly.
In practice, I have used 3 Pre Pay data packages, tethering a Skypephone S2, and even in a '3' dongle, with no problems at all, for at least the last 2 or 3 years !
You should also note that the only alternative for unlimited data on pay & go - Giffgaff's googybags of £10 upwards - also exclude tethering officially, but I have used the current free data on Giffgaff in my netbook for a year and a half roughly ( as well as in '3' mobiles. )
However, you are correct in pointing out the official position, as I said earlier.
( I shall reply to your PM tomorrow now. )