She was fed up and upset, and did something that a lot of us would like to do because she reached a breaking point. I think it was amusing, and even if it wasn't the "right" thing to do, I'm not going to sit in judgment of her. I can understand that there are those who don't like what she did, and everyone is free to state their opinions. However, I find the moral outraged expressed by the one poster to be out of line with the degree of the wrongdoing. This leads to the belief that ripping down a poster is equivalent to breaking into someone's home to steal, and I find that kind of thinking to be dangerous. I also do think that what Kathleen Huggins did could fall under the guise of civil disobedience if she believed she was doing something for higher moral purpose. It may not be right in the eyes of the law, but people have different beliefs as to what is right and wrong, and we're not necessarily going to agree that what Kathleen Huggins did was wrong (or right), no matter how it is put to us.
During the last election, Nevada had an initiative on the ballot to make gay marriages illegal, and many of my neighbors had signs in support of this measure. I found these signs insulting on several levels (they said protect marriage and showed a stylized diamond ring), and I was tempted to pull some of them up, but I didn't because ultimately I decided that was not a good way to respond. If I felt like abortion was wrong and I pulled down a poster for a family planning clinic that offered abortion services, I'd feel that I was doing something right even though I might be breaking a law.
On the subject of civil disobedience I have to say that Rosa Parks broke a law when she sat where she did on that bus. That it was an unjust law is a view shared by many people today, but people have different moral standards and not everyone would agree that it was unjust. To plenty of people at the time, what Rosa Parks did was wrong if for no other reason than she broke the law, and some people may think that there is no excuse for that. And other people didn't think there was a problem with that particular law, I'm sure. Sensibilities and moral standards have changed, and we now easily see that what she did was standing up for a higher moral purpose. What the cancer clinic was doing in displaying a formula poster is legal, certainly, but that doesn't mean it is morally ethical. I'm not comparing Huggins' actions to Parks' in intensity, but I think that their intent and reactions were similar. Given that I can make that comparison, I can then understand that there are those who would do the same type of comparison and say breaking the law is wrong no matter what. I acknowledge that.
The clinic personnel were perfectly within their rights to call the police. I would be surprised if the police or the courts thought this was an offense worth pursuing, however.