How can we complain about the "scammer" label when we engage in this kind of abuse of terminology ?
The 6% is a return, but not on your
investment.

Yep, fully agree. The term 'ROI' has been wrongly attributed to the Masternode service reward annual return for years.
The term 'ROI' simply does not apply for 'savings', it's called interest. Nobody says they have a 1% ROI on their savings account. Staking is most similar to a 'savings' account. The MN service reward is comparable to staking.
It's not malicious though, it's a knowledge/understanding issue.