said by globus9991:2 - Legal reasons. I am not a lawyer but had my share of regulation analyzing. I just read the current copyright law and can find no place, whatsoever, where it indicates that an ISP *must* keep logs of any kind all the time, short of mandated by a judge (e.g. a specific IP is being sued by a rights owner and then it must be logged and existing logs retained). So, there are no valid legal reasons to keep ANY log whatsoever on an ongoing basis as a routine operation.
Try checking under other laws. There are legal reasons that aren't related to copyright that might require the keeping of IP logs.
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