I guess you're also using \usepackage[french]{babel}? The French module in biblatex (french.lbx) formats last names in small capitals by providing this "local" definition of the \mkbibnamelast command:
\protected\def\mkbibnamelast#1{\textsc{\textnohyphenation{#1}}}
To restore this command to its "global" definition add the following to your preamble:
\DefineBibliographyExtras{french}{\restorecommand\mkbibnamelast}
In biblatex.def the global definition is:
\newcommand*{\mkbibnamelast}[1]{#1}
You can redefine this via \renewcommand*. The code in \DefineBibliographyExtras is hooked into citation commands and bibliography drivers. So the French definition will simply be restored to the "current" global definition.