With my first Tesla road trip coming up next week, I think this policy is a good idea, and I hope it helps with availability of congested CA chargers.
I think the 5 minute grace period is plenty, because in practice you will actually get much longer. As an example, take one of my planned stops: Harris Ranch - arrive 20% - depart 68% - charge time 29 minutes
I'm not silly enough to lower my charge limit to 68%, because any extra charging I do at Harris Ranch will save me time at Gilroy. So, my grace period doesn't even start ticking until my car hits 90%, after an estimated 54 minutes. 54 charging + 5 grace = 59 minutes, an effective grace period of 30 minutes. That's more than enough time to pay for lunch and get back to the car. If I really wanted to push it, I could raise my charge limit to 100% and get another half-hour of grace. I don't expect to be there anywhere near that long, but if I somehow blow through all that extra time, I deserve to get charged for messing up other people's travel plans.
The only time the grace period is actually limited to 5 minutes is when you're trying to charge all the way to 100%. I understand certain trips require the full range, so I wouldn't put any limits on people charging to 100%. But, after doing a slow range charge at a busy supercharger, the least you can do is leave promptly afterwards.
