A flamer won't do anything but make an Elemental mad, but it'd be the perfect weapon for unarmored infantry. But power armor infantry, as far as I can tell by browing Sarna.net, doesn't even exist in the Inner Sphere until the Clans bring it with them and show the Houses what they've been missing out on, so it'd be (wrong, but) logical to mentally categorize them as infantry for weapons usage.