That's because it's used for the crates in the Sunken Ghost Ship, and if it kept its behavior you could spawn enough mushrooms and capes to potentially crash the game. Why Nintendo didn't just use the switch bottoms and arrange the graphics accordingly is beyond me. Actually, that combined with how the switches and blocks are among the last objects (and the outside corner is the last), combined with the fact that they use an OW GFX file and a pressed P-Switch is in GFX17, it seems that the switch blocks and switches were the last tiles coded into the game...
Let's milk Sunny Milk. Then she'll have enough money to fund Sunny Milk Real Estate.
Everypony's digging with a shovel
mushroom block in ghost house
Extras
I should have something witty to put here (even if it's just to update dated info), shouldn't I?
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Why not use a custom block that always spawns Mushrooms instead?