cjrgreen: It wasn't "traces" of horse DNA; it was 29% horse meat content. Somebody dumped a lot of horse meat in the supply chain, and nobody noticed, or if anybody noticed, they didn't care or were ordered to ignore it, no matter what that pride in traceable content suggests.
Even if the pig content was mere "traces", this is probably not reassuring to kosher or halal customers. But the reason this is a big deal is that if this is all the control they have over their meat supply, they could just as witlessly have passed through contaminated, diseased, or spoiled meat.
Tesco's been having a hard time recently. They've been pinched from both below and above on UK sales, and their "Fresh 'n' Easy" grocery chain in the US is a total loss. This doesn't make things any better.
This is basically what I was saying as well, not sure what your problem is, other than I wrote "traces" rather than saying 29% of horse/pork meat in a beef burger, but that was because I'm sure everyone in this thread is familiar with the news. The main issue people have is not that they ate 29% horse meat - which in itself does not necessarily cause any harm - but rather that it damages the image of "quality, traceable national stock" that most of these retail chains are trying to build up, and can understandably raise further questions and suspicions about the content of the retailers' own products.