This is a tough one, because I can understand where both sides are coming from. That being said, if there is something wrong with constructing bridges over rivers, the group should focus their effort on preventing new ones from being built. They are just wasting time, money, and making people upset by targeting bridges (especially historic ones) that already exist to be destroyed. It might also be better for them to target bridges that have fallen into disrepair or pose some sort of threat to public safety.
Although, I suppose they are seeking attention and trying to make a statement by targeting a bridge many people are familiar with.
