They did that to work around a problem where people were paying $.01 just to get a steam key.
Adding a game to a steam account "validates it" and this can be abused in certain scenarios. For example, steam sometimes has events where you can enter contests, and people were creating hundreds of fake entries for almost no money.
Edit: just to be clear, for other bundles you can pay $.01 and get the DRM free games but not the steam keys (which require a payment >= $1). Here, since the games require steam, the minimum price is $1.
Adding a game to a steam account "validates it" and this can be abused in certain scenarios. For example, steam sometimes has events where you can enter contests, and people were creating hundreds of fake entries for almost no money.
Edit: just to be clear, for other bundles you can pay $.01 and get the DRM free games but not the steam keys (which require a payment >= $1). Here, since the games require steam, the minimum price is $1.