For sure, I understand the government there wants to both control and verify capital outflows, but it could still be quite easy to achieve these aims. Alipay / wechat could just setup an online passport verification system where you need a passport and chinese visa to register, accept transfers of cash through paypal or some other payment network which foreigners have access to, and just make it one way ie. cash can be moved into an account, but not out of it, or can only be withdrawn into RMB cash through a partner bank. Or alternatively setup a weekly limit on withdrawals back to paypal to some amount which complies with their capital controls.
I understand the argument that wechat and alipay don't care about foreigners when it is a very small percentage of the market, but they still have a chance to pickup quite a lot of incremental revenue for very little cost if they can execute it. Politics not withstanding, inbound tourism into China will probably grow over time. They also have a chance to expose foreigners to the convenience they offer, and take on visa/mastercard in foreign markets. In my last couple of visits to Sydney I've seen alipay and wechat signs everywhere... I bet there is a market in western countries for these services, especially for people who are averse to credit cards