Well first off I don't believe they'd put her to sleep just for one accident. They'd have her go through a behavior/temperment test first and there they would notice she's fearful, NOT aggressive. That still doesn't improve her chances of being placed, but will let everyone know she did not bite out of aggression.

You're completely right though, you NEVER drag a scared animal by the collar and not expect something like that to happen! that lady seems like such a moron.