There are two posts in the thread Kazzymodus linked. One pertains to consoles (which includes 3DS), the other pertains to mobile. These are the policies that we use in regard to discussing mobile/console modding on this forum (TCF).
The primary reason for the policy as it pertains to mobile is that the mobile platform has suffered a bit with stability. When the first mobile versions were released, a few people quickly posted some modifications, and other people began using them and then reporting problems with the modded versions. This was starting to become an issue as far as providing support, so this policy was put into place to help focus on the issues within the official versions.
Of course, what you do with your own device is your business, not ours. We only ask that you not use this forum (TCF) to promote/explain/boast about such modifications while these policies are in effect.
I think we'd all like to see a point in the development where at least some of the freedoms enjoyed by the PC versions are available to console/mobile players, even if it's as small as being able to share worlds or seeds. We're just not at that point now.
I don't want to get too far off the main thread topic - if you have questions about the modding policies, you can always PM me or any staff member about them.
Also a reminder from the opening post that these features are still experimental, may be risky to your data (worlds/players), and potentially may not be available in later versions.