By making the existing legal restrictions easy to understand
by humans
by lawyers
by machines
Mixing licenses makes the data less interoperable
Mixed data as free as the least free component
Contract unsuitable for data
Difficult to track
Difficult to enforce
Apply only between consenting parties aka "privity of contract"
Legal requirement potentially a disincentive to use
Converge toward the public domain
Maximize legal interoperability
Allow commercial use
Science is innovative when the market is able to convert scientific outcome into affordable, commodity products (zipper, ballpoint pen, aspirin, penicilin)
Don't impose share-alike
"Do as I do" leads to an impasse because I can't foresee all possible future scenarios
Choose PD or CC0
Public Domain (PD) works best when data are completely free of copyrightable elements
CC0 (pronounced "CC Zero") is more appropriate when copyrightable elements and facts are mixed. This is the more usual case
Norms not Contracts
Encourage citation through community norms, not through contracts
Contracts are hard to implement, expensive to enforce
Science is governed by norms
(PD or CC0) + SC Norms help converge data toward public domain