Fantasy is ripe with tropes of 'sacred spaces' that don't exist in temples: Sacred Groves, Ancient Burial Grounds, Protected Forests. Are these not Hallowed Ground? Are you telling me the Iconic Archetype, the one with a 'transcendent union with nature' is unable to protect these 'holy' places or create sacred places that undead or evil are unable to profane?
The way I see it, druids are unconcerned with alignment stuff. Nature covers both the soothing rain that brings life to the land, and the hurricane that tears life from it. It's both the mother wolf nursing her cubs, and the same wolf savagely tearing into its prey. Some druids are good and some are evil, but neither is inherent to being a druid, and thus their magic doesn't much care either way.
Another thought, though this one's more of a rationalization: sure, there are Sacred Groves and such. But those are natural (well, more or less) phenomena, not created ones. You don't have druids deciding "This would be a good place for a sacred grove" - rather, sacred groves exist and druids are interested in protecting them. This is unlike the temples clerics use, which are built by man.