I *really* hate to ask, but has anybody got a link to a site telling us
that loosestrife is illegal? I'm sure not finding it.

I don't. I grew up a farmer in Montana, and as such learned at my daddy's
knee about noxious weeds, including garden flowers that escaped cultivation
and caused problems. Loosestrife has a beautiful flower, and it also takes
over swamps and wetlands. Big no-no.