Actually that isn't the case. Any China Mobile SIM (at least those purchased in Shanghai) can have the add-on global roaming package. You can enable it by simply calling 10086. For prepaid users they may require a minimum balance (¥200 last I checked). It's also limited to one month initially so you have to keep calling back to reenable it. After a while though they give up and just leave it on all the time

I was in the US last month and found that T-Mobile is now China Mobile's preferred roaming partner and the rates are quite reasonable: US calls at ¥0.59 per minute, receiving from anywhere/calling China at ¥0.99 per minute, and ¥0.19 per message for texting (data hasn't changed at ¥0.01 per kilobyte though). For comparison when I was there last November it cost ¥2.99 per minute to call locally and ¥4.99 to receive calls on AT&T.