One suspects the stores in France (and I've seen the shrink-wrap when I was in Germany too) want to avoid people doing exactly what you did. Granted, I did exactly the same thing as a teenager in the US. In big stores like Barnes & Noble the staff mostly let me read in peace, but in smaller stores the staff/owner would make some comment like "This isn't a public library! Buy it or get out!" after a few minutes of reading.