Mushrooms can spawn on the sunlight places when you discovered their landscape at night. This happens in every biome, but mostly in swampland and roofed forest. They also generate on the tops of trees! Normally, mushrooms shouldn't be right under the sky, they must have some cover.
How to reproduce:
1) Explore a world at night.
2) Find some mushrooms on a top of a tree or right under the sky.
3) Remember the coordinates.
4) Create a world with a same seed, tp to these coords (you will find there are no mushrooms).
5) Create a world with a same seed at 1.7.4, set night time & tp to these coords (no mushrooms again).
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I just discovered that mushroom generation depends on the time of day. If you explore a swampland at night, there will be lots of mushrooms, but when you try to explore a swampland at daytime, you won't see any mushroom right under the sky. This glitch can't be intended.
Re-resolved as duplicate of MC-49798 due to more detailed reproduction description.
Mojang, namely Dinnerbone, said that they like such things in the game (he said that for grass in caves, but I'm sure it's also meant for things like this)
Also, otherwise the swamp would lack alot of mushrroms.