The principle here behind the decision whether or not to engage ground targets that are not air-oriented is one of, as you note, the protection of the population. The UN resolution calls for a ceasefire on the part of the Libyan pro-regime forces. When they are visibly refusing to abide by that ceasefire, when they are going and rearming themselves, and we can observe that they are doing this and they are then firing on the population, or firing on other forces--in other words, violating a ceasefire--then we have a fairly clear understanding of whether that particular location is one that is--
On March 23rd, 2011. See this statement in context.