Asking me to look at code on github is not that different from just sending me all the code (other than the fact that you aren't cluttering my inbox with it). What code do I look at? Why is it interesting to me? I understand your point about having a discussion about your code, but you are again asking me to find those interesting bits that are relevant to the position I want to fill. You are familiar with the code base already - it is not difficult for you in your cover letter to indicate that I might be interested in x,y,z in project foo because they demonstrate a,b,c, and I can find the code on github. Now I know what to look for, and I am very likely to go and look for it. That may lead me to explore some other things and will lead to an interesting interview. Even if I don't consider the code interesting and relevant, it still lets me ask about why YOU thought it was. If you don't point me to that, I may just flip through a couple of methods and never find anything that interests me enough to talk about it.

And for the record, I don't like "puzzles" either - even if I know the "trick" to the puzzle, sometimes my brain doesn't work right and I can't think of it after a long day of interviews. I would much prefer to be evaluated on how I work in a real environment, seeing my real code and my approach to real problems as opposed to impossible problems whose solution hinges on an obscure bit of semantic parsing in order to arrive at the solution. But as an interviewer, I don't have unlimited time to evaluate candidates so if you want me to do something, it helps to make it as easy as possible for me to do it.

In the end, I don't disagree with you, but I can understand interviewers that don't make the effort if you don't also make an effort.