Because copyright infringement is not the same as theft and they are quoting the accusers.

BBC's style guide is to quote whatever literal statements, even in the title

the other reply explained BBC's style guide, but even if it were not a case of quoting, note that the US Supreme Court has ruled (Dowling v. United States) that copyright infringement cannot be considered stealing under the law regarding interstate commerce in stolen property, and in other rulings US courts have barred plaintiffs from using the terms "piracy" and "theft" to describe copyright infringement, so in the US at least, the terms do deserve quotes.

Not my opinion, just the facts.