@Fatma34 It is totally unfair to accuse this host of racism just because she asked for insight as to whether there would be a reason in Muslim traditions for them to keep the curtains closed all day for 2 days.
She said that she asks guests to open the windows to ventilate the house and they weren't doing that. What you may not realize is that hosts have had guests who were doing drugs in their home, smoking in a non-smoking listing, snuck in a dog, and all kinds of other nefarious activity. You have no basis on which to assume that she would not have been suspicious if the guests weren't Muslim.
Most onsite hosts would wonder what was going on if guests didn't emerge from their place for 2 days and the curtains stayed closed regardless of the guests' race, religion or anything else. Personally, I would be concerned that they might be ill.
However, knocking on the door after she had seen and spoken to the guests was unnecessary and an invasion of privacy, I would agree. But I see that as simply a nosy host, which she comes across as. There are some hosts like that, unfortunately, who want to micromanage every aspect of a guest's stay and have a ton of rules.
Believe me, I understand issues of discrimination, and hosts with racist or other similar prejudices certainly shouldn't be hosting on Airbnb. But it's presumptuous to say this host wouldn't have been suspicious had the guests not been Muslim. Nothing she wrote would bear that out.