Presumably they were so called because boon originally meant gift, and they were thought to be a gift to the lord, but were actually required.
The online glossary I found called the serfs "manor workers," which I find distasteful in the same way as lots of southern plantation houses-turned-museums use the word "servants" instead of "slaves." Or they say "Joe Schmoe the III built Tippity Top Mansion in 1742..." when what they really mean was "It was the enslaved African Americans owned by Joe Schmoe III who built Tippity Top Mansion..."
Eugh. I got off-topic. But thanks for posting, rolig, I had never heard of subbotniki before.
This reminds me of the practice in the Soviet Union whereby people were expected to do unpaid "volunteer" work on Saturdays at least once every few months. These days were called "subbotniki" (from "subbota" - Saturday). There were also "voskresniki" (from "voskresenye" - Sunday) which took place on Sunday.