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Store the environment in "global" table. Make a new environment. Point the __index of the new environment at the global table. Make __newindex of the new environment a function that throws an error.

Congradulations, you have metaprogrammed lua to only set globals explicitly. Good job!

It is due to the implementation used for lexical scoping. With 5.2, I actually consider it wrong to think Lua code has a global scope and it has no global get or set VM op instructions, there is however an environment. The environment's default name being _ENV and _ENV can also be local. _G is normally, in 5.2, an entry in _ENV that points to back to _ENV.

Because the original intention for lua was as a simple extension language for non-professional programmers[1]

[1]: http://www.lua.org/history.html