Unless that letter has been trimmed by Abby or her editor, she's made an interesting assumption. The letter writer does not say how the date was altered. The couple could have been secretly married before the actual wedding, but didn't want to cheat their parents (or themselves) out of a big wedding with all the trimmings. Perhaps they feared that the parents and/or wedding guests would be upset if they knew the wedding was a fake.
I have seen wedding dates in obituaries. "While volunteering at her church, Janet met Bob Smith, the love of her life. They were married on February 7, 1975, and had 30 happy years together until Bob's death in 2005." However, I more often see "Janet was preceded in death by Bob, her beloved husband of 30 years."
It may also depend on the purpose of the obituary--if it is to be a paid obituary in a newspaper or online, of if it is more of a biography to be given to attendees at a funeral service. For the latter, people often do put in a lot of detail.