I've been caught out a couple of times when sending a message to someone - there is automatically a tick in the box 'allow this member access to my tree' and you actually have to delete the tick if you don't want the person to see your tree.
This is probably what has happened.
I think it would be much more honest of them to reverse the process so that you tick the box if you want that person to see your tree - but then, it's within their interests for as many people as possible to have access to as many trees as possible.
I still get a few decent contacts from this site per year so think it's probably still worth the hassle. However, I'm really getting fed up with the number of people who either just don't bother to answer messages (why bother to put their tree on in the first place?) or simply send a 'give me access to your tree' message without any 'foreplay' (sorry, it's the best analogy I could come up with!

)
I can see why people are leaving the site in droves.
Jill