Cultural question to Brits as this happened in UK. Do people care if their dog attacks someone or is highly agressive to humans in UK?
By what I've heard from my friends, dog owners often put the blame on attacked/harassed passers-by, they don't feel accountable for their wrongdoings (i.e. not using leash and not paying attention to where their dog is and what is it doing).
Those overly negative comments often come from USA. I've never had any major problems with self checkout in Europe and I generally go there as it's faster and you don't have to race against the cashier. Of course some chains have worse self checkouts, some have better but overall many people like it a lot. Even some older people who are not tech savvy use them.