what is the point in function access to globals being read-only?

[Skaperen]

if the global variable exists, already, a read access gets that value instead of its default if not found. for assigning, it does not do the same. i'm wondering about the reasoning for that, since it could do the same. it's just that the default is different. most other languages don't have globals at all, so it's hard to find a comparison. most use some kind of class or function thing to do that. maybe this is why it is considered to be unpythonic to use globals?