In that case, I agree with your logic entirely. To me, any time you say, “Here's the drop” or “Here's the start time for the limits,” on some level it's arbitrary, because we're campaigning all the time.
I can see there being a different formula applied to third parties, because their behaviour is not necessarily campaigning all the time in the same way that a political entity does. However, I take your point: what stops the millionaire from dropping the money in the day before, especially when we have the fixed election dates, when you know it's coming and you can plan it?
That's also the issue with foreign donations. There's nothing wrong with a third party accepting foreign donations, as long as they're not used for election purposes. It's the same thing. If you have that drop and it's prior to the regulated period, that's just part of the funds that are in the organization's bank account. Those are your own funds, and you can use them.