Two guys have been learning the Talmud together for 20 years. One of them says to the other one, “You know, my daughter is getting married next month. We’ve been learning together for years, and so I’d like to honor you with being one of the witnesses at the ceremony.”
"I'm sorry, I can't."
"But why can't you?"
"I'm not Jewish."
"What do you mean? We have been learning together for 20 years. You're as frum as I am."
"I enjoy the intellectual stimulation."
"But we learned that a gentile that keep shabbos is chayav misah! "
"I never kept shabbos. Every time I was ready to leave my house, I put a key in my pocket."
"But we have an eruv here!"
"I don't hold from that eruv."
This idea, that a non-Jew cannot keep Shabbat, is from this week's parasha:
ומה ראית לומר גוי ששמר את השבת חיב מיתה? אמר רבי חיא בר אבא, אמר רבי יוחנן: בנוהג שבעולם מלך ומטרונה יושבין ומסיחין זה עם זה, מי שבא ומכניס עצמו ביניהם, אינו חיב מיתה? כך השבת הזו בין ישראל ובין הקדוש ברוך הוא, שנאמר: ”ביני ובין בני ישראל“, לפיכך, כל גוי שבא ומכניס עצמו ביניהם, עד שלא קיבל עליו למול, חיב מיתה.
The problem with *this*, however, is that the reason doesn't work: Shabbat is a sign between ה׳ and בני ישראל *because* ה׳ created the world and rested on the seventh day? Creation applies to everyone!
So
I would read כי in both these places (כי יצר לב האדם רע and כי ששת ימים עשה ה׳ את השמים ואת הארץ) as "despite", not "because". ה׳ promises to never curse the world *even though* the nature of Man is to do evil, and Shabbat is an אות *even though* ה׳ created the universe, and
the implication of the דרש is that keeping Shabbat is *forbidden* to human beings; לא ישבתו. The Jewish people were given a special dispensation to observe it, as an אות of their ברית עולם.