Why do these people willingly subject themselves to this form of slavery?
Blessings from Jehovah!
It sounds funny, but really, that's what most of them will think.
The guys who work on Bethel construction projects have fully quaffed the kool-aid. They take seriously and literally the idea that (1) the absolute most important thing that any being in the universe can be involved in is "sanctifying Jehovah's name" and (2) the Watchtower Society is absolutely the earthly manifestation of the organization that is "sanctifying Jehovah's name".
Thus, if the WTS gains a cool $1 billion dollars from real estate sales, and the windfall was purely and entirely the result of thousands or millions of hours of donated labor, well, hey, fantastic! What a spectacular witness! How sanctifying! And to think my blisters & callouses contributed to this historic event! I'm so overcome I think I'm going to cry!
I don't have enough fingers & toes to count the number of people I know who would react just that way.