Yes it is legal ... some 1099-misc box 3 forms can still be considered self employment depending on the circumstances.
Box 3 is a "catch all" and used when the amount entered doesn't belong on any other form/box and it is up to the reveiver to put it on the tax return in the proper place.
Read the instructions: https://www.irs.gov/pub/irs-pdf/i1099mec.pdf
