Charged with careless driving in Ontario?

My husband was recently charged with careless driving in Ontario and we don't understand why. He was in the process of making a left turn into a parking lot with his signal on when the car behind him skidded and slammed into his side door with such force that his door couldn't be opened. The entire side of the car was wrecked. He only walked away with a small bruise on his head and some scs on his body but actually blacked out for a few seconds. Can anyone explain to me why he was charged when the other driver was obviously speeding and not paying attention or else he would have seen the left turn signal on my husbands car. The speed limit on this particular street is 50 kilometres and the impact alone shows that the other driver was going much faster than that. Never mind the tire marks from his car showing that he was going much faster than he was supposed to. There were no witnesses, just the two drivers. Please, if anyone can explain this to me I would greatly appreciate it!