Porting and supporting a game in a profesional way is much more work, than doing is as a spare time project. As spare time project you have no responcebillity for fixing bugs and get everything working as smooth as possible.
Also it not like I didn't considered to spend some of the money I would possibly earn with additional contend, but the first thing that a project like this would need to do is to finance itself. The developer need some money to life, he needs the development equipment (a development kit), and after this is granted, everything additional could be spendet.
Also its very likely that bug fixes for the ported version will go back to the original project, and this way it would help the original developer too.
By the way, I would have asked the developers before I would have started porting a game, so why would it be a problem of the result would have been free, and the user would have only needed to pay for additional contend?
Also like you said, to get games like teeworlds run smothly on a system like the 3DS you would need to optimize it, and this could also need very much work, and sometimes you even need to rewrite complete parts of a game to make this possible.
Generally I was thinking much about doing it, but I it would have been a very big risk, so I won't do it this way, but maybe I will try to do this in my spare time. So don't expect me to release something in the near future, but maybe some day in future there will be a teeworlds or a other port on the eshop