Most likely because China could care less. Contrary to the image China projects about being best friends with North Korea, they are pretty much as sick as NK as the rest of the world. China has lost billions in investments to NK, and if you've ever lent $20 to someone and never had it paid back, you can begin to imagine how they feel.
That being said, China does actively enforce the border, but there is a myriad of different political reasons for this. The "legal" reason is that they have pacts with North Korea in relation to this issue (hence why escaping NK aren't granted refugee status by China). Some of the other reasons include the fact that many Chinese citizens are just as paranoid about North Koreans coming across and taking Chinese jobs as Americans are about Mexico.
Others have already replied to your thread and also pointed out that other reasons may include China have an equal investment in getting information out. China probably isn't interested in the trade aspect so much, as it's entirely black market and therefore next to impossible to regulate.
Unfortunately I don't have an direct citations to back up the above, but I am basing most of my information off of "Nothing to Envy: Ordinary Lives in North Korea" by Barbara Demick. It's an excellent read. Most people try and draw parallels to first world countries with 1984, but the sad truth is North Korea could be considered source material.