Funny that you'd call it terrorism actually. I could see where it would be treated as that. Certainly wouldn't make it any easier to stop- it still is very difficult to figure out who was behind it and bring them to justice unless they are sloppy or confess.
Actual DDoSing is done maliciously, for the sole purpose of disrupting normal services. It usually comes with a high price tag for the victim due to bandwidth overages and lost profits due to the downtime- and having been shot at before there I can honestly say it's a horrible feeling and an unexpectedly large bill that can reach bank breaking levels. Typically DDoSing relies on a botnet, but it has also been possible by distributing malicious software for end users to voluntarily participate by using. A good example of this is the infamous LOIC, which has a long history with Anonymous for making websites dissappear.
Now making hype enough to get a whole bunch of people to visit a site, that's not at all malicious. Such is typical of the so-called slashdot effect, capable of throwing a DDoS-like flood of traffic known to cripple unsuspecting servers. That's completely okay, and only happens when someone ends up unexpectedly popular. Online protesting would fall into this category, as actual people are all actively there visiting a site near-simultaneously to view its content or contribute their opinions. If the server fails under the load, then the operator was unprepared for the public response.
Attacking a site by DDoS also usually affects other sites near it as well. There are far more permanent ways to take down a site than to simply attack it, most of them relying on legal procedures. And I honestly can think of very few reasons where a DDoS might be considered a legitimate course of action. If it's illegal let the lawyers do their thing. If it's legal, sorry they have as much of a right to have it as you have to hate them for it.