Like many other 'value added' features for high reputation users, I'm suggesting that while the API should be freely available to every SO user (Keys can be handed out on the SO user's own page if you want to restrict API access to just SO users), I know that if everyone accesses the API once a minute 24/7, SO will get swamped.
There will necessarily be restrictions in place to limit usage.
I'm suggesting that these limits are relaxed for high-rep users. They are the ones likely most interested in comprehensive 3rd party interfaces, and the most likely to use them significantly to enhance their already excessive use of SO.
Further, it would artifically slow down the API roll out. Deploy them to high rep users first, see how the system responds, and lower the rep level needed for certain API functions as the system is tested and able to accept heavier API usage.
While I'm not a fan of restrictions, I understand that some restrictions will have to be put in place, and rather than requiring everyone to request a high-usage key directly from the SO team (which will tie up resources that might be better spent elsewhere) decide on a rep level where it makes sense to automatically provide that access.
As a side effect, if you have access levels higher than 10k (say 25k if you want to start off with fewer than 93 maximum possible users) then you'll give others more incentives to go beyond 10k. Even if they don't intend to use it, they will have a goal post
feeding their addiction encouraging them.