I've noticed one thing about most blockbuster movies these days: most of them are rated PG-13. G & PG are now considered too childish by the "important" movie going demographic (teens & young adults), so are usually not big blockbusters (with a few notable exceptions). R rated movies block out the teens (a significant portion of the "important" movie going demographic). PG-13 is the perfect rating, as you satisfy the "important" movie going demographic and are able generate the most revenue. Just look at the three big blockbusters from May: Spiderman 3, Shrek the Third, & Pirates. They all have the PG-13 rating. That's no coincidence
So, to bring this back to the original topic, having Live Free or Die Hard rated PG-13 makes very good sense from a marketing point of view, but maybe not from an artistic one (though that is open to debate, which everyone above have discussed rather well). Also, I think it likely that we will probably see an unrated version on DVD later on. The only negative thing about that is that it wouldn't be a theatrical release.