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Because there's historically very few studies about dog breed temperament and those which exist are bad. So when you end up with 8 deaths in a year by XL Bullys and people start to say it's a problem, it's a stretch to use the lack of good studies as evidence that actually all dog breeds are equal. And even then by her own admission pitbulls in the USA are vastly over represented in the data and she still says they're not a stand out problem. The part I don't know is why people feel they need to defend an obviously problematic dog breed against all available evidence.
I'm not sure what's controversial or agenda driven about that though. If the evidence is weak then sure, but what do you think her agenda is besides presenting her own research?
I don't see this as downplaying the danger of certain breeds, but highlighting the problem of aggressive dogs in general. I'm very happy for strict enforcement on bans of dangerous dogs breeds, but if that doesn't prevent harm from other breeds then that's pretty important.