To be more precise, some logos are not copyrightable. From http://www.copyright.gov/circs/circ1.pdf:

"Several categories of material are generally not eligible for federal copyright protection. These include among others: ... Titles, names, short phrases, and slogans; familiar symbols or designs; mere variations of typographic ornamentation, lettering, or coloring; mere listings of ingredients or contents."

Every legally protected logo I've ever seen is trademarked either at a state or federal level.