How this usually works is that samples are relevant to specific experiments. For example, maybe you're testing how some particular genetic modification affects how cancer cells behave in response to some drug, or whatever. These vials of cells would be what you get halfway through such an experiment, after you've treated the cells but before you've had a chance to analyze them. Postdocs and studentships often end midway through projects like this. The raw value of the materials is low, and it might take a postdoc days to weeks to reproduce them, depending on complexity. There is also an extremely high chance that the cells are completely worthless, i.e. that the treatment was botched or inconclusive. If the student hadn't taken the vials, the project might well have been abandoned, due to lack of manpower.

With that background, the morality of "smuggling" these cells is a bit of a gray area. On one hand, they're just incomplete work that the student wants to finish elsewhere. Keeping them is just like me keeping my notebooks when I move, even if I haven't finished solving the equations therein. On the other hand, if the project is finished elsewhere, credit won't properly flow to the affiliation where the work was started. In theoretical physics, we really don't care about this (I just finished a project where 2 members changed affiliation midway through), but in biology there's the cost of facilities, and the local expertise that gets you the raw materials.

So there is an issue here, but I certainly don't think it's worth the hysteria that will unfold in this comments section. I think the real issue is that a lot of people on this site are disgusted at the idea of Chinese people learning science. But to us scientists, the spread of knowledge is a virtue. After all, the end goal of this student would have been to publish in a journal -- which people from any country can access.