100 credits is not that much. And at the same time it gives players a reason to log in every day. Once they’re logged in, they will probably play a game or more: Good for the player population.
Players that never spent anything in the shop can now earn credits. At some point they will maybe want to buy a 1200 bundle because they already saved up 700 for example. They buy the remaining 500 from 343: brings in cash for 343 from a player that without the free 700 would’ve never bought the 500.
Also, players that are ashamed to wear anything from the shop can now wear it, because there was always a possibility they bought it with free credits. It removes the shame for both players who buy a lot in the shop and players who never buy anything from the shop. And it will probably bring in more cash for 343.
At this point it is pretty much needed to reward players to log in every day. Just my opinion though. 
A different route is to reward less credits, but to reward players just for every day they log in. So 10 credits per day for example, but they don’t need a streak.
Anyway, like I said I only see upsides to this method of rewarding players to log in every day.
- Players are rewarded for their dedication to the game.
- 343 earns more cash.
- Player population will most likely go up.