Nonces and rapists would almost certainly go directly to a crown court (with a jury) so it will be a different judge. So you are not comparing 'apples with apples'.
Most people involved with the rioting have been charged with violent disorder. This carries a maximum sentence of 5 years. Most people i've seen have been sentenced to 2 to 3 years, so nowhere near the max.
An Act to abolish the common law offences of riot, rout, unlawful assembly and affray and certain statutory offences relating to public order; to create new offences relating to public order; to control public processions and assemblies; to control the stirring up of racial hatred; to provide...
www.legislation.gov.uk
Most rapists don't seem to even be taken to court, and when they do they often get off, but that's a different issue.