Probably the writer thought that clerics could only summon good aligned monsters with the spell. That or they didn't realize clerics got summon monster at all.
Regardless, it needs fixing. I like the idea of adding a bunch of specialized spells to the cleric's spell list... but I'm not sure there are all that many that aren't already on it.
If you want to beef it up a little and give the cleric some benefit, allow the cleric to cast summon nature's ally to summon fire creatures. That gives these creatures to the cleric 1 level earlier, in general.