They are designed to be safe and to code, when used as directed, they weren't used as directed.
There is no evidence that Pirelli directed the teams with regards to mandating the tyres are run on the correct side and to specific cambers.
Infact, Pirelli were well aware of the teams doing this, and said nothing. If they felt this was dangerous they would have told the teams not to. The fact they didn't and the fact that they have only starting mentioning this after a bunch of failures as a reason, shows that they did not think there would be an issue.
Its all very well pointing fingers at the teams after the fact and blaming them, but its too late. Pirelli thought the tyres were safe, Pirelli thought the tyres continued to be safe when used as the teams were doing so, but they turned out not to be. Therefore, not fit for purpose.
If Pirelli had no idea about how the teams were using the tyres then I would take their point. The thing for me is that they knew what was going on and didn't stop it, but then after some failure are trying to say "well what you were doing was unsafe". If that was true, they would have said something earlier.