He knew who she was -- well, obviously. He wasn't the one having a total freakout followed by a complete meltdown on the side of the street. And the total meltdown also explained why he didn't see her as a threat.
Before she even realized what she was doing, she was evaluating him as a threat. Sitting, not standing, not too close... Could have done whatever he wanted before she even noticed him. So not really a threat. Plus she remembered him; she'd sort of enjoyed talking with him that time when she was threatening all the vampires. It had almost been like he was trying to help her make her point. No straying from the topic, no unintended consequences. Very straightforward. Model vampire. Not quite as good as the one who'd dis-invited her from some social thing, but right up there.
Besides, she'd given up being the Slayer. "It's a free sidewalk," she said, not exactly graciously. It felt like a weird thing to say, like how could she even get caught up in threat evaluation and sidewalks, or identity and being the Slayer or not -- how could this be something that even mattered...but it sorta did.
And it sort of didn't. She ought to be angry about being caught like this, but what difference did it really make, compared to everything else?
"You're not going to bother me," she said, half reflexive warning, half truth. "But you might be here a long time." All truth.