Why are some academic research datasets not available to people who only have freely available email addresses?
For example, ImageNet is not available to people only have freely available email addresses:
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If the owner/hoster of a database wants to provide that data only to a certain group of people (e.g. researchers, EU-citizens) and wants to keep the administrative overhead as low as possible, he has two possibilities:
open the database for everyone, put it under a specific license and a trust that the data is only used according to that license
provide only access to people with email addresses from a specific domain (i.e. official research institutions)
Verifying each registered user personally would be to much work.
The second way is a common concept for scientific databases. Some databases have a two step model (I think, ECMWF uses it): you can register with every email address but you cannot access all the data.
Often, there is no bad faith behind these restrictions. The server capacities might not be sufficient to provide the service to each possible user or the data (pictures, texts, ...) are provided by third parties, who only want to provide those data for free for a certain audience.