You can always lower land on the border, but you can never raise

[Buster's Uncle]

It doesn't matter that the other faction has higher land that will be effected if you lower the altitude of a square at the border and not if you raise it - you can lower but not raise.  Makes no sense.

[Geo]

Perhaps it's more likely the humidity levels of the tiles will change when you raise land, and therefore lowering wasn't included as a 'hostile' actoon towards a neighbouring faction?

[Buster's Uncle]

Just as likely that you're lowering land on his east side and making him drier.