The purpose of a sit-in is to stage a non-violent, disruptive protest which forces a response, typically being arrested, from a business owner or government. If there is no response because, for example, the business owner is too shamed to respond then either the disruption continues until the protesters demands are met or the protesters eventually give up. If there is a response, such as use of law enforcement, then there is the opportunity for the protesters and their cohorts to get the message out, raise awareness, and gain sympathy and support from the public at large.
Anonymity changes the equation entirely. Sit-in protesters blatantly flout the law and beg to get arrested, so as to demonstrate their conviction for the rightness of their cause and to gain publicity. Anonymous DDoSers aim only to disrupt, whatever message they have is lost to the public because nobody can know for sure who the people responsible (4chan also apparently DDoS'd minecraft.net for a while, what was the purpose of that? wikileaks itself was DDoS'd for sometime, what was the purpose of that? it's hard to say because these are just anonymous attacks rather than actual, disciplined protests). DDoS participants don't expect to get caught, let alone to be arrested, as witness the reactions of the folks in the main article.
An anonymous, or attempted anonymous, denial of service attack is more akin to throwing bricks through a storefront window or arson. It is disruption completely unmoored from personal sacrifice, civil disobedience, or the context of a cause.
It is thus properly a more violent form of protest than a sit-in and we have all the more reason to despise such things because of it.