Is the DLC locked to one 3DS at a time or can it somehow be tied to the cartridge?
It's locked to the 3DS. Good in that other people can borrow your 3DS to get access to some DLC they haven't purchased (e.g. FE:Awakening). Bad if you're using a cartridge and switch systems ever without moving accounts.
yeah that's my main problem with it. They were very clear in the beginning that they were not going to go that route. It's the precedent this sets that bothers me because while it isn't bad value proposition for Mario Golf there is no guarantee about how future releases will pan out.
If it helps, the game was probably ready for release months ago (it was originally announced for Summer 2013, if you recall), and they likely held back the release in order to finish the DLC to sell close to release.
So instead of "release the game and 60 days later release first DLC pack", it's "hold back the game 60 days to release game and DLC pack together." The Nintendo Difference™.