I agree, and think even an evil character can have a "pure heart" under the right circumstances. In that context, it's less about good/evil and more about what the character's intentions are for activating the sigil. The Linear Guild certainly had unpure hearts when it came to the sigils, so they were excluded. The Stick, however, had good intentions (or lacked evil intentions), so they were good to go.
That said, I can't imagine Thog having any clue or intention as to why they needed to touch the sigils. So I may be wrong about the whole evil thing.