No, it does not mean that circularly polarized antennas like the Penetrator are illegal. LPFM stations are allowed to use circularly polarized antennas, as specified in §73.816(a) of the FCC rules. This regulation permits LPFM stations to use antennas with horizontal only polarization, vertical only polarization, circular polarization, or elliptical polarization. Therefore, you can use circularly polarized antennas like the Penetrator without violating the rules.