Because to her a Marxist bag is just a fashion accessory:

“I sincerely apologise to anyone I may have inadvertently offended,” Diaz said in a statement. “The bag was a purchase I made as a tourist in China and I did not realise the potentially hurtful nature of the slogan printed on it.”

This is the problem with wearing Marxist clothes and accessories as a fashion statement, unless you understand what the history behind the symbols you run the risk of committing a faux pas when you run into those who have lived through the devastation of a communist dictatorship.
(via)