Many years ago it used to go down for technical reasons.
Now it is purely PR/Marketing. They get massive traffic spikes whenever it goes up and it instantly results in thousands of web pages going up with free PR. Why would you give that up ?
-Outside of Apple employees, you would be hard pressed to find a person with better inside information on Apple than John Gruber
-Additionally, in the tweet, he is corroborating another Apple journalist's similar claim
-Saying it is a technical flaw reflects somewhat poorly on Apple, and Gruber isn't exactly one to go out of the way to claim Apple is doing something poorly without a real reason to say so
-"It is for the PR!" always struck me as pretty weak post hoc reasoning. Sure, it drums up some interest in the tech blogs, but it also means the store just does not work for a period of time for everyone. Including people who don't care and just want to hand Apple money. I presume Apple wouldn't go out of their way to frustrate customers and possibly lose money.