True no-kill rescues are becoming fewer and fewer these days, thanks to draconian 'dangerous dogs' legislation and the compensation/legal action culture we have these days. Sadly, they can't risk being sue and shut down and not being able to save the lives of hundreds or even thousands of dogs by allowing a dog to live that then goes on to attack someone. Sadly one bite is often enough for a death sentence, regardless of the circumstances under which it happened.
The alternative is keeping dogs alive, essentially in solitary confinement - which is still not foolproof and obviously not in the best interests of the dog. I used to volunteer for the Dogs Trust way back when they were the NCDL. They had two long-term, unrehomable dogs and only one member of staff was allowed to go in with them (haven't a clue what happened when they were off sick or on holiday). On my first day no-one told me this and sent me over to that block to start cleaning out the kennels. I started with the first kennel in the block - as you would - only to have a member of staff turn up a short while later and go deathly white. Apparently I was in with a dog (enormous GSD) that was known to be highly aggressive and had already bitten badly on numerous occasions. Poor lad, he was beautiful and for some reason showed absolutely no sign of aggression to me, in fact he was retrieving his ball for me when the staff member turned up. They were gobsmacked and said he usually went ballistic if anyone went anywhere near his run. I think I was just lucky that, because I didn't know his profile, I went in there cheerfully and confidently and as luck would have it, he responded positively to that. The fact is, it could have gone horribly wrong, I could have been badly attacked and the dog was clearly extremely lonely and suffering from being kept isolated for so long ... no one is a winner in these situations, not the dog - or the rescue organisation. It's rarely a decision taken lightly, at least not in decent rescues that do a lot of work to rehabilitate and appropriately rehome as many dogs as they possibly can.
As bakingtins said, I don't blame the rescues, I blame the over-breeding, the appalling amount of cruelty towards animals, the treating dogs as toys and throw-away possessions, the complacent irresponsibility of owners that fail to socialise and train or learn even the basics of how to communicate with and meet the needs of their dogs and then dump them when they grow up, are no longer cute or trendy enough and haven't miraculously trained themselves and all the other irresponsible things far too many dog owners do.
