This is one of the grey areas of the law where it can seem ridiculous. If someone is clearly committing a crime on your property, why should you be held accountable? The most extreme example I can think of is someone who was breaking into a factory who fell through the roof and successfully sued the factory owners for having an unsafe roof.
However, the issue comes down to what is reasonable behaviour
if the person detained or hurt is innocent. Suppose in the roof example, someone had been up there for an innocent reason (e.g. they were a contracted workman, a dad had gone on the roof to retrieve their kid's kite, that sort of thing). In those circumstances, it would be right that the owners were sued. Again, suppose that some have a go hero detained you because you were innocently passing the scene of the crime and you looked suspicious. How would you feel about being physically restrained, threatened with being hit, etc? You would (rightly) have grounds for suing the person.
I know that it makes the blood boil when police appear to be bovine over this, but the point of the law is that it must unambiguously protect the innocent. If this means the guilty get away with things on occasion, then that is the price to be paid. The alternative is far worse. How would you like it if vigilante groups started patrolling the neighbourhoods assuming police powers when they saw anyone they thought was suspicious?
Having said all of this, I've tried being a good citizen on a couple of occasions and the police have treated me like a time-wasting moron. For example, I once found £200 in notes tied up with a rubber band in the street. I handed it in to the police station and they treated me like I was a gullible idiot ('oh well, I'd better do the paperwork' was the response I got; actually, I got to keep the money in the end because no one claimed it, but that's besides the point

). Or on another occasion I saw a group of chav teens, none above the age of about 15, driving a brand new Rover pretty erratically along a road, pulling over onto the verge so they could swap drivers. I phoned the police and got the question - 'why do you think they could have stolen the car?'.