Their tacit allowance and failure to even halt any USN vessels in areas they claim repudiates their supposed claim of said bodies of water of regional interest.
Why even try though? Should they have entered into a confrontation with the US ships, China would have shown the region what it's trying not to, in that it's not engaged in a "peaceful rise" and is committed and willing to engage in confrontations with larger nations, not just the confrontations between it and smaller powers like Vietnam.
China did nothing, which domestically might be considered weakness, on forums like this too where nationalism is in abundance, but it shows they know restraint and can use it. They shadowed, but did not confront US ships, ramming or interfering with one was never a realistic prospect to begin with and the Chinese aren't stupid enough to risk a confrontation anyway, but they've now demonstrated this to the remaining SCS nations... where or not they believe China's show in the face of past actions is another story.
Still, China had nothing to gain from halting the US' passage though those waters, it would have demonstrated a commitment to aggressive acts, a lack of commitment to international law (ironically, it was China's failure to adhere to WTO standards that lead to the TPP being formed, so they've a historical reference to note when deciding whether or not to challenge the international order when more than the US is involved) and shown its less powerful, more frightened neighbors that the US, and not China, is right in its stance towards the other.
China played its card right, the alternative would be deadly. It's worth noting that aggression or rational action, the region still has its own historical examples of China's actions towards them. I suppose no action, peaceful or otherwise, will change this dynamic for Chinese-SCS relation. China could have made things worse... it didn't.