This is really interesting research. But I'm a little surprised a research group was able to tap the entirety of a large university campus's internet traffic. Is that common?
The research is by teams at those universities whose networks were observed, not some outside group. There’s an entire section on “Ethical Considerations” in the paper that details what they did (and did not) collect. Like any research involving human subjects in some way, their methodologies were reviewed by their respective university institutional review boards (not that those are necessarily perfect).
There are plenty of organisations who will let you tap Gbps network links for research purposes and a small fee.
Network data isn't supposed to be secret, and in fact I wish more networks would publish samples of data that passes through the network so users inadvertently not using encryption can be alerted.