At the same time, IF China could actually use the islands to enforce shipping regulations (tariffs) they would actually be able to control the area.
I think it's a little more complicated than that. The Chinese don't want to do anything that might disrupt shipping, because they make a lot of money off that too. Furthermore, they need the oil that comes through there. It's really more about bullying their neighbors. For example, practically within swimming distance of Subi Reef is the Philippine held island of Thitu. On Thitu is a small detachment of Philippino Marines and a rough airstrip usable by C-130s. There's not much else there. I think there's plans to put some luxury resorts out there. I'm sure the snorkeling and scuba diving are great. If the Chinese wanted to cut them off, all they'd need to do is turn on their SAM radars, have their fighters start capping over the island and announce they're no longer going to tolerate the Philippino military presence on their islands. The Philippino government can't do anything to stop them. They can either agree to leave or starve.
That (and other things) puts the Philippines government in an awkward position, because if the Chinese government came to them and said they'd like them to accept unfavorable terms in an agreement to do oil and mineral exploration in the vicinity of that island, they can't really say "No," because the Chinese government doesn't really have to ask. Thus they'd be able to slowly erode the Philippines (or the Vietnamese, or the Malaysian, or the Brunei, or the Taiwanese) effective claims to the islands, without firing a shot, and at the same time exploit them economically, treating their concerns regarding economic development as subordinate to their own.
The US position in the SCS is neutral. While the US government supports the UN Convention on the Law of the Sea, and argues that the Spratly Islands represent an excessive claim on the part of the Chinese, it doesn't necessarily support anyone else's claim either. The government doesn't really care whose islands they are, so long as whatever solution is reached is arrived at peacefully. You're right, the US could probably steamroller those installations fairly quickly if pressed. That's not really who they're aimed at, though. They're really about coercing the other claimants to the Spratlys into accepting Chinese dominion over everything inside The Nine-Dash Line.
Given that, it's actually kind of a complicated thing trying to figure out under what conditions the US would actually intervene in the SCS. I don't think it would happen, unless either the US or one of their regional allies were actually attacked by the Chinese, which they might choose to do if the Chinese government felt that one of the claimants was insufficiently submissive to their interests, or if it was perceived as easier than strong arming the regional powers into giving up their own rights. Basically, one of the claimant nations would have to instigate something, or the potential US intervention would have to be perceived by the Chinese government as weak. In that case, the US would probably honor its agreements. The thing is, neither the claimants, nor the Chinese, nor the US really wants a war in the SCS. It's kind of an "everybody loses," situation. So the interesting question in my mind, is how does the US support its allies interests and deter Chinese aggression without firing a shot? I think that's really hard.
