We need some context and please name your source. The word isn't necessarily negative, of course: it depends on how you view whatever it is that people are being 'active' about. And it doesn't have to be political activism.
It's a matter of opinion. If you feel demonstrations, protests, and other forms of political action are alway negative, then for you, "activism" will have a negative connotation. For other people, that is not necessarily the case.
Activism does imply a little more than writing a stern letter to the newspaper, however it takes many forms. It is that form that determines if "activism" is negative, positive or neutral. Activism has the nuance of "doing something that will create publicity in the furtherance of a cause."
In your example "community activism" activism is moderated by "community" and is positive - it brings people together for mutual benefit. It is unlikely that the participants will set fire to a bank or riot in the street - they will simply take a greater interest in, and spend more time working for, the cause, marching with a banner, organising a talk on the matter, try and lobby local politicians, and perhaps report their efforts to the local press, etc. This is positive.