It doesn't cost any money to defend against a suit that no judge would accept.
To allow a suit based on the assumption that the FEC was wrong and Forbes must have known so is the kind of insanity that gets a judge removed from the bench in even the most conservative jurisdictions.
So no, there's absolutely no valid excuse for Forbes to use the word in this case.
It really doesn't. Lawyers on retainer are on paid no matter whether they have anything to do. That's what being on retainer mean.
It costs nothing to ignore an unlawful legal request, at least not when you already have lawyers on retainer to do exactly that. A publication the size of Forbes ABSOLUTELY do.
There's no legal or economic downside to ommitting "alleged" and it still sends the misleading message that she might be innocent, which could feed into her false martyrdom scam and actually help "earn" her a lot more money than the fine cost.
In conclusion: there's no potential downside to NOT spreading false doubt like that and there's a ton of potential downside to doing it.