This is from a few years ago, but it explains a lot about the Ursack and the factors in play in deciding whether to allow its use. It's the 9th Circuit Court of Appeals decision from Ursack's lawsuit against Yosemite, Sequoia-Kings Canyon, and Inyo: [PDF]
http://cdn.ca9.uscourts.gov/datastor...9/09-17152.pdf
In a nutshell, several field tests found it inadequate (in its various iterations) because:
- Bears were able to rip open the sack and receive food
- Bears were causing significant damage to trees that the sacks were tied to
- Bears were able to bite into the sack and contaminate food with their saliva
- Bears were able to crush or tear the aluminum insert, leaving metal bits in the food
The last two factors are especially troubling, since they suggest that hikers would either go to extra lengths to scare off bears who might damage/destroy their food, thus putting themselves in danger, and that if the food were damaged or destroyed, it would be more likely for a hiker to dump it. So the considerations aren't entirely about whether the Ursack simply manages not to be penetrated.
Maybe things are different this time around, but this is an issue with a fairly long history, and Ursack had consistently failed to meet expectations even standards that they agreed to be tested by. The people who have decided not to allow it so far aren't doing so arbitrarily even though they might come to a different conclusion than other land managers have and they're generally people who are on our side: they want these areas to be wild, accessible, and safe.