I may be able to answer that (from a Canadian perspective at least, I'm not sure where Tiercel operated his).
I seem to remember it as a flight manual restriction, because I think I can still recall the temporary pages in the book. If memory serves, it resulted from an accident on the James Bay hydro development (around 1980?) where a 500 flamed out in falling snow and crashed. The investigation revealed the Auto Relight, which was located on the lower console between the passenger's feet was not turned on. The investigators also concluded that the Auto Relight could not be seen by the pilot if the center seat was occupied, and therefore the pilot could easily miss it if it were not on. The result was the restriction.
As a solution to the problem, most Canadian operators then relocated the Auto Relight to the outside of the instrument panel on the copilot's side, where the pilot could see it. If your 500 already had this or an equivalent mod applied, there would be no restriction. I think Tiecel may have his dates a little off, by the early eighties this would have already been done, I believe.
I haven't flown a 500 for 8 years, but that's how I remember it. Hope it sheds some light.