Comment by IAmBroom

It does make sense; the poisons in morels (and many other "edible" mushrooms) are highly volatile. Heating them in a pan drives off the harmful compounds; heating them more gently in a large volume of liquid captures the compounds.

There's a variety of mushroom that has killed in the US, but is reportedly sold in Scandinavian markets. My theory is that Scandinavian recipes specify pan-frying or drying them first, and the unlike USian skipped this step.