I wouldn’t judge this as an unusual rule.
Exodus 20:10 (KJV)
But the seventh day [is] the sabbath of the LORD thy God: [in it] thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that [is] within thy gates:
My guess is that he is honoring the idea that he will not require a stranger to be on duty so that he may enjoy the premises.
Personally, I buy no food on my Shabbat. I don’t even buy a cup of coffee nor trade a coupon for one, so that no one has to make it. Even though it is already made for others.
I had to drive 700 miles during my Shabbat this week in order to keep a schedule that I had no control over, but I at least honored it to the extent that I didn’t purchase fuel, food, nor coffee.
edit: I was obviously answering while
@Philip was.
Not debating, just supporting his understanding.