Lengths and substitution in L-systems
L-systems, are more about general : Abstract rewriting systems rather then to concrete application.

Agree, a wide range of L-systems applications is in computer graphics (generating nature, textures etc), while from mathematical point of view, they are still, the same model.

As stated in referred by you L-systems definition :


  Using L-systems for generating graphical images requires that the symbols in the model refer to elements of a drawing on the computer screen.


Coming back to your question 


  Yet in (...) implementation, even where a symbol is not defined, it will still be rendered as a full length. The question is, why?


Answer: Because that's the way, how those implementations' authors decided to map symbols into rendering commands.

You can do in your implementation any other mapping... and still both of them can be same type of system - L-system.

It's all about defining isomorphism between systems behind implementations, or homomorphism between those systems and graphics they generate. If you will make your own implementation, that will draw symbols differently - you will indirectly define your own homomorphism.