I agree that this is a big issue. In my experience, the problem has to do with two main issues:
1) National legal frameworks + the national understanding of those legal frameworks.
2) Legacy practices and customs from legal teams in government which have not been necessarily exposed to the culture of openness.
Regarding the first issue: as a general rule, the interpretation of Mexico's Legal Council (Ministry level institution) and our National Copyrights Institute is that the Mexican government cannot 'license' the use of government public data, since it is by default a public good 'owned' by all Mexicans. In this sense, our Free Use Terms serve as a clarification mechanism for data users, so that understand that data can be freely used, reused, and redistributed by anyone, anytime, anywhere. From this legal interpretation, it follows that all public data published by the Mexican government is free to use unless it is otherwise stated.
This brings me to the second issue: although all government public data is tacitly open by default, this is not necessarily understood by users if its not stated straightforward in government websites - as it is in datos.gob.mx -.
To address this issue, we are working to mandate the use of our Free Use Terms the default terms for all public data, in all government websites. By doing this we are in fact changing the equation so that data is clearly 'Open by Default' (from the legal perspective). This action has been greatly influenced by the conversations around GODI, and we hope to have it ready by end of the month/ early June.
Hope this example can be useful for UY and other countries.