They would be useful to terrorists. But they are more useful to people with peaceful purposes, by several orders of magnitude.
Any use that a terrorist could find for them can also be found by a security auditor role-playing as an attacker. If a terrorist could find a weak point in the infrastructure that may be exploited, so too may a defender, who may then devise a countermeasure, rather than an attack plan.
Those countermeasures would not necessarily appear on the maps. For instance, there may be a motion-sensing surveillance camera watching a locked gate on an access road to a reservoir for a municipal water supply. Such measures are sufficient to foil impulsive attacks, and planned attacks very often generate suspicious activity reports and probable cause for investigations well in advance of the planned event.
In theory, the maps may be a security risk. In practice, they cannot substitute for in-person investigation of specific sites, and most important infrastructure elements are at least partially protected against impulse crimes and accidental damage by chain-link fences, concrete barriers, motion sensors, and cameras. The typical terrorist candidate doesn't have satellite-guided ICBMs--they have to actually go to something in order to attack it. And they are constrained by the need for their attacks to be significant, public, and obvious.
Think about the implications of making the locations of these sites secret in a free society. Would the existence of plants be hidden from locals? Should it be illegal to talk about aggregations of plants and debate them?
If the existence of the plants is too dangerous for civil discourse, maybe we shouldn't have them at all, to play devil's advocate.
