I'm all for holding a hotel accountable, but in this instance I have to say that I think you got more than you should have been entitled to.

Bed boards (as they're called, it's not just some random piece of wood they found in the alley) are commonly used in the hospitality industry for customers who need a firmer sleeping condition than the standard mattress might allow.

While it is quite funny and idiotic that whoever placed it there thought it should go under the sheets and not the mattress, and that is warranting of an apology and maybe a few goodwill points, short of placing the bed board under your mattress I'm not sure what else you expected the hotel to do? Perhaps go buy a special mattress just for you?

Here is a prettier version of the same thing.