The oddities of the way children think

So today we went to the Toronto Science Centre for friends of ours son's birthday.
We had an awesome time.
William didn't stop talking about it after we got home.

We put him to bed late because he fell asleep on the drive home (slept from 545pm-700pm).
Thirty minutes later we hear him yelling for one of us.
I go up and climb into bed with him (he's in a loft double bed).
He is clearly upset.  When I finally get him calmed down enough to understand what he is saying I realize he is agonizing over the fact he got separated from us briefly in the Science Arcade.  And clearly it scared him.  It was only a minute or two that he was away from us.  He had moved onto the next exhibit and hadn't realized Andrew and I were not following him.  He immediately turned back around and came back to where he had last been and found Andrew and I looking for him.

Apparently this scared him more then he let on at the time.
I spent the next hour walking him through what to do if he did actually get lost.

Someone, not Andrew or I, has started to teach my son that you never talk to strangers.

This pisses me off because it is seriously wrong and messed up.
How is the kid ever supposed to make friends if he can never talk to someone he has never met through me or his father?  How is he supposed to ask for help with something if he can't talk to someone I or Andrew have not introduced to him and why has it suddenly become taboo to be polite and greet people on the street/in passing etc????

I have taught William that he can talk to people he doesn't know.  He can greet people (Good morning/afternoon/night, How are you etc) and he may tell them his name.
He has also been taught he can never go with someone Andrew and I don't know and that he doesn't know regardless of whether it is a kid or a grown up.

So I helped William work through what to do if he gets lost anywhere.
I told him that if he got separated and went back to where he last saw us and we weren't there to stand still.  We can't find him if he keeps moving.  I then told him if someone asked him if he needed help or was lost he could answer yes.  I told him if they tried to get him to go somewhere he was to say no and that if the person wanted to, they could stay with him while he waited for us to find him.  I also told him it would be a good idea to tell that person our names (which he knows well).
Though I did tell him that if he got lost in a grocery store that going to the person working at the cash register was also acceptable.
We repeated this several times before he calmed down and was able to relax enough to sleep.