One could argue that you'd only hand out account details to real friends, whereas a link in a PM could be sent to any old Joe. If it wasn't possible to change passwords, would content sharing with strangers happen, or would it be confined to people you trust? So perhaps they aren't making it difficult enough to ensure the sharing isn't further than friends sharing their gaming experiences on PS3?
Edit : In response to Patsu, it's not just price sensitivity, which on £5 games isn't a huge issue, but also a general broadening of tastes. I have shared my purchase of flOw with friends who wouldn't have bought the game but have enjoyed it a bit. It's an experience they'd never be willing to pay for which they'll only experience as a freebie, but which doesn't impact sales of the game because they wouldn't have bought it anyway. This ties in with what some developers have said against DRM - those who pirate a game would never have bought it anyway so you're not losing a sale. Now I don't think that's accurate, but the theory carries some water, and one cannot equate a PSN game shared to being a sale lost.
Edit : In response to Patsu, it's not just price sensitivity, which on £5 games isn't a huge issue, but also a general broadening of tastes. I have shared my purchase of flOw with friends who wouldn't have bought the game but have enjoyed it a bit. It's an experience they'd never be willing to pay for which they'll only experience as a freebie, but which doesn't impact sales of the game because they wouldn't have bought it anyway. This ties in with what some developers have said against DRM - those who pirate a game would never have bought it anyway so you're not losing a sale. Now I don't think that's accurate, but the theory carries some water, and one cannot equate a PSN game shared to being a sale lost.