Most CA's are completely fine in the high 60s (so long as it happens gradually). Some SA's are more needy in the way of heat, but some others actually NEED water to cool off for weeks to a few months of 50s+.
I had wondered the same as well. Went out of town last weekend and the house temp had dropped significantly (as expected when the high is 7*). My tank stayed warm enough, but my texas was definatly sluggish until I got it warmed back up.