@james no, it is a technology problem. People will use the technology how the technology implies it can be used.
Allowing people to bookmark and search up toots implies that they aren't impermanent, when they are very likely to just vanish or become inacessable at any moment.
@jameschip but implication is not intent. A screwdriver implies it can be used to open paint cans, but if you try you might bend it.
If people learn posts are not permanent, they've learnt a useful lesson about data online. If it's not on your machine, it can just go away.
I don't believe in protecting users from every dumb mistake, it just makes smarter idiots.