OSM is a database of geographic facts ("There is a road here. It is called Main Street. It is a primary road." etc.) Copyright of databases is non-obvious, so they are switching to a click through database EULA thingie.
Copyright of databases has on several occasions been overturned
See Feist V. Rural
"The constitutional requirement necessitates independent creation plus a modicum of creativity. Since facts do not owe their origin to an act of authorship, they are not original, and thus are not copyrightable. Although a compilation of facts may possess the requisite originality because the author typically chooses which facts to include, in what order to place them, and how to arrange the data so that readers may use them effectively, copyright protection extends only to those components of the work that are original to the author, not to the facts themselves. This fact/expression dichotomy severely limits the scope of protection in fact-based works."
Whether or not the street data is copyrightable in light of this decision is definitely sketchy; the tiles etc. clearly are since they have some modicum of creativity.
And remember that OSM is a global project, that works in many countries, with different interpretations of copyright law. You don't want it to be open in some countries, public domain in others etc etc
