I don't know the legal side of things, but in terms of the current protocol, the default fixed mixin count is the current implementation.
I.e., if you type
transfer gingeropolous@payxmr.com 500 1
it will send me 500 1 xmr with a mixin (or ring size) of 3.... and nowhere in the command is there a 3 specified.
Let me test that.
transfer gingeropolous@payxmr.com 1
For URL: gingeropolous@payxmr.com, WARNING: DNSSEC validation was unsuccessful, this address may not be correct!
Monero Address = 46aRPgXEgqf23G2VU5fy4oeKBGpU6uXSv57CkXKrz4EbDwDeh573QQhZYeyjriWAMUhNoBHMUwkGV2A2ppWwAx4JT9HNJ9h
Is this OK? (Y/n) y
Money successfully sent, transaction
Wow, how does that work (the sending to your 'email' part)? Pretty cool!
Edit, the default 'mixin' is 4 by the way (when / will we be changing that incorrect term?)
http://moneroblocks.info/search/aabb4052eec0b2097246c2bd4ff2d5421a1ffdb086bbe42d7318500db22280b4