Not all foul play is equal. The IRB recognises that - it does not insist on a card for a "reactionary" high tackle.Clarification 9 of 2004I don't believe "foul play" is that ambiguous... It's anything covered by Law 10 (which includes dangerous play).
If a penalty try is awarded as the result of a player unintentionally offending, the player, as well as being liable to cautioning and temporary suspension or send off, can be admonished by the referee.
Once againI think we have to interpret the laws sensibly rather than try to do so literally (and we cannot guarantee an agreed meaning anyway).