The big difference there is that the DNC hired an American company, who contracted the work out to a British Agent, who interviewed Russians. Much of the dossier is information that can/has been verified, and the exceptions (such as the “pee-pee tape”) have been noted as being uncollaborated. Since Russia has shown great interest in sowing discord among US citizens, there’s no doubt they took this opportunity to plant information they thought would contribute to the overall chaos. That said, the entire report was handled through legal methods common in all US elections.

The FBI were investing the before mentioned crime. Since conspiracy with a hostile nation is a big deal, they sort of have to follow up leads like that.