why do most PTC sites have a "bux" on their names?

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i was once interested in PTC. but i lost my enthusiasm since those sites i joined in are somewhat just scams. but my disappointment is a different story. i observed that most (if no all) PTC sites always have "bux" or "box" with them. some more popular are neobux, gagabux, and the rest. so it got me wondering if "bux" is a requirement to identify them from other sites. or maybe bux has an entirely different meaning which would make it appropriate to be appended to a PTC domain name. does anybody have an idea about this? any explanation is highl appreciated. =)