Would you voluntarily work in a country that invaded and taken over yours?
Would you ask the same about Russia recruiting Ukrainians?
Some are.
Some people also want to feed their families and not be unemployed.
P.S. A large number of Ukrainians moved to Russia. Including the ones who wanted to reunite with their relatives. Taiwanese have a lot of relatives on the mainland as well.
Citation very much needed.
This means people that have Ukrainian citizenship, which includes lots of families that were on both sides of the border. It does not mean people who were residents of Ukraine before the war, and decided to move to Russia. It does include the kidnapped children, and surviving people in occupied cities.