[mschuster91]

For this, one could introduce a "quick freeze" scheme: providers don't store anything (or, if needed for billing etc., delete after 7 days).

Only if police knocks up and tells you "we might need the data from IP address x.y.z.a in the foreseeable future", you store the requested data on secure material.

Then, police goes to court and gets a formal warrant for the data, which the provider then needs to provide the data to the police.

[eurleif]

The need is to identify a subscriber given an IP address they used in the past (to commit a crime). Knowing who is using a dynamic IP address now doesn't necessarily tell you who was previously using it.