I'm not sure what the reason is. There's a hotel across the street from me, and some people were looking out of the window, instantly they had 5 lasers pointed at them. Someone was pointing a laser at me too.

The protesters want to avoid their faces being photographed. It's a reasonable thing to want, although I must admit it is pretty annoying.

Same reason protesters disrupted traffic by blocking roads and holding train doors open during rush hour to prevent people from going to work, disrespect the national flag, etc.

Some of them are protesting for reasonable demands (even universal suffrage is not too unrealistic a demand for Beijing IMO but any dose of separatism is definitely a no-no) but increasingly it looks like the majority are students venting their anger because they found themselves in a bad economic situation and have a long summer break to spare.