Are you 100% sure the story is even true. I've met Tawakul Karman at a woman's empowerment conference where hijab and niqab were discussed and this incident was never brought up. In fact, it doesn't sound like her stance on hijab or niqab at all.

Also, from wikipedia:

Karman started protests as an advocate for press freedoms in her country. At a time when she was advocating for more press freedom, she responded to the Jyllands-Posten Muhammad cartoons controversy in 2005 by writing: "We are not to call for tyranny and bans on freedom."[20][24]

She stopped wearing the traditional niqab in favour of more colourful hijabs that showed her face. She first appeared without the niqab at a conference in 2004.[11] Karman replaced the niqab for the scarf in public on national television to make her point that the full covering is cultural and not dictated by Islam.[25][26] She told the Yemen Times in 2010 that:[11]

Frankly I think this is propaganda that some over zealous idiot made up and put out.