Sociology is a science because we use the scientific method of hypothesizing, testing, theorizing, re-testing to see if we get the same results, quantitative and qualitative analysis of data, and doing it all over again to better understand groups.
However, like our retarded half-brother non-science called psychology, sociology also falls into the trap of having no clearly defined "natural" symptoms or cures of a social disease. Everything in the social sciences are based on perception of researchers rather than hard objective evidence of social disease.
For example, you see a group of kids in an alley, you ask 5 different sociologists and psychologists what is going on and why the kids are in an alley. You'll likely get 10 different answers. There are no solid laws for interaction in sociology and psychology.
Whereas if you go to an engineering lab, you ask an engineer why hot water moves from a hot body to cooler body. Every engineer will give you the same answer because there are concrete and universal laws of thermal dynamics.