Some dogs react to different things differently. My parents dog (Jack Russell) doesn't react negatively to anything while walking (that I've experienced) but will bark and nip at bicycles, so I always kept him close after finding out the first time.

I have never come across a rollerblader in my life in Canberra, so I have no idea how my parents dog will react to a rollerblader - I would presume badly due to the unique noise rollerblades make hitting pavement.

If someone has a dog who only reacts to rollerbladers (and hasn't experienced it yet) and has shown no signs of aggression before, its difficult to know that uncommon circumstance to react properly.

Dogs are ultimately unpredictable animals; I would recommend crossing the street or avoiding interacting with them while on rollerblades in the future because it may be a new experience for dog owners they haven't needed to prepare for to "control" and teach properly.

Yes it's the dogs and owners fault, but sometimes its unpredictable.