Even if it isn't a revenue stream it can still have value to them, it doesn't have to be for "fun". This may simply be one of their ways of acquiring metrics.

As for it not appearing in their TOS, the "risk", it isn't unusual for the left hand to not know what the right is doing. Those drafting the legal text may not have known that this particular analytics system was being used.

There are many arguments for why this was overlooked, and why it exists, without it having to be direct revenue.