21st Army group (British, Canadian and Polish formations) encountered far fewer Concentration camps due to their position on the Northern flank of the advance. They would have been responsible for Bergen-Belsen and Westerbork, and several smaller subsidiary camps.

Bergen-Belsen was surrendered to 21st army group by Himmler, following his efforts to liquidate as much of the surviving population as possible. There have been allegations that several guards there were subject to summary execution following the discovery of those orders & this camp, allied troops excusing their alleged conduct by stating SS guards under martial law refused to aid in the clean-up work, claiming that it was degrading & dangerous work; At the time, a typhoid epidemic was sweeping the camp. In this context, it would be very easy arrange for the death of many of these guards without fear of repercussion.

Due to the poor record keeping by 21st Army group in this area, either pictorial or written, direct allegations are predominately based on hearsay and oral history. There is however fairly substantial evidence that SS guards were treated very poorly by their captors, as this popular image shows. http://media.iwm.org.uk/iwm/mediaLib/37/media-37149/large.jpg?action=d&cat=photographs (British troops allow hungarian SS guard to rest in newly cleared mass grave)

In the context of the situation, and the wider allied reaction, it would be best to assume guilt and collusion in the summary execution, deliberate negligence and degrading treatment allied forces handed out to captured SS guards.

The majority of the concentration camps (and almost all of the dedicated extermination camps) were in Eastern Europe — primarily in Poland — so the Soviets were the first to reach them during their advance westward.

Several of the largest camps — particularly the Polish extermination camps — had already been closed, however. From 1942, a dedicated effort (Sonderaktion 1005) to conceal evidence of genocide in Poland was carried out.

In most other cases, camp authorities evacuated the camps prior to the arrival of Soviet forces, specifically in order to avoid the predictable reprisals.

For example:

• Treblinka and Sobibor: were both closed and their sites cleared in 1943 following (separate) prisoner uprisings. Bełżec was also closed in 1943 as part of Sonderaktion 1005.
• Majdanek was hastily evacuated in July 1944 in the face of Soviets' rapid advance through Poland.
• Auschwitz-Birkenau was evacuated in January 1945, prior to Soviet forces' arrival; the remaining prisoners were either killed on the spot or forced to undertake 'death marches' to camps in Germany.

All of which is to say: there were relatively few Soviet liberations where camp staff and guards were still present. The reason there's more writing about reprisals by US/UK troops is that the camps they liberated were primarily those within Germany itself (Bergen-Belsen, Buchenwald, Dachau).

Edit: just remembered I wrote a comment on another question a while ago about accounts (and evidentiary problems with those accounts) of Allied reprisals against SS prisoners, which might be of interest.

I don't know if this is an acceptable comment due to the source, but the only thing my grandfather EVER said about the war, was that after they came across their first concentration camp, they didn't take a lot of prisoners for a while. This was very, very unlike him to say anything like this. It was actually very chilling and still stands out from when I was a very little kid.