You're right, it doesn't make a lot of sense and that answer, while true, is a bit of a cop-out because it's a normal FEATURE of relational databases. I believe it's called "referential constraint" in the relational db world. Of course you can't just delete the rule and that's all. Believe it or not, Cisco has already solved a nearly identical problem with the inspection rules (and multiple other places in MARS). When you change an inspection rule, it actually COPIES it. The old rule is left unchanged so any records (i.e. incidents) with foreign keys pointing to it are not orphaned or left pointing to a rule that doesn't match. Also, try deleting a user who has cases assigned...you should notice that you have to re-assign the cases to someone else. That's because otherwise it would leave orphaned records. IMO, the correct answer from Cisco should be "we just haven't added that functionality yet"...not "it's so we don't leave orphaned records or for non-repudiation".
My 2 cents, and probably grossly oversimplified, add a column to the inspection rule record that stores the last incident created by it. When deleting a rule, is the incident still in the dynamic data?...yes...can't delete or ask user if okay to delete incident too.
