I think the article misstates this. They’re normally legal spots, but at the time of the special event, they’re not legal.(Usually temporary signs are placed a day or two in advance.)
Even if it is, that’s not sufficient to inform car owners to move their vehicles. Where I live, all public parking (whether you have a parking permit or park in spots which don’t require one) has a 72 hour limit. If you put up temporary No Parking signs with less than 72 hours notice, you’re guaranteeing many people will have their cars towed inappropriately.
Based on the stories here, not not sure this is happening.