It's bad when crossing the border. Rumor says it's due to traffic analysis by the GFW. I cannot find any source to prove or disprove that though.
Data localisation regulations are certainly one reason for foreign companies to set up local infra, but IMHO the real reason is that it's the only way to provide an even remotely tolerable user experience to users in mainland China. Aliyun is basically AWS for China and is very easy to get up and running, and getting an ICP license wasn't nearly as hard as everyone makes it out to be (got one in ~2 weeks from application).