I wouldn't dream of asking permission for something when I already HAVE that permission. That seems unctuous to me.
What will you do if someone says, "no, you can't have my publicly posted picture?" when that person has no right under Ancestry's T&C to withhold it. I'm not a lawyer, but this is simple contract law, no?
If someone has uploaded to Ancestry a copyrighted photo from a third-party website, THAT can be addressed because it clearly violates Ancestry's T&C if the copyright holder has not granted permission.
By the way, it makes no sense to compare "Miranda warnings" [criminal law] to Ancestry's T&C [contract law].....apples and oranges. That much I learned from sharing dormitory space with law school students. ;-)
I like to write a public thank-you comment to the effect of "thank you for being kind enough to upload this photo and make it available to others; it gives me a real sense of my ancestors," etc. Usually I also like to say things like "your ancestor was a handsome man" [if I can say that without crossing my fingers behind my back - lol!] and other such observations.
This seems straightforward to me. I have not yet had any complaints, but then again I'm always certain to affix the image to the correct identity. Wouldn't want great-great grand-father the candlestick-maker wrongly identified as a horse thief. OR VICE VERSA. ;-)