Though it both simplifies things for their explanation and increases their numbers , so I can see the temptation.
And I don't think you can credibly deny that in actual practice - in what I gather they call real life - humanism equals secular humanism equals atheism. How that state of affairs came about I don't know, since strictly speaking neither humanism nor secularism
per se imply or mandate atheism. The great majority - I'd say almost all - of theists are secularists, for instance,
sensu stricto. But a quick once-over of the BHA's manifesto/aims/whatever leaves you in no doubt that they're predicated squarely on a non-religious, atheistic worldview.
I doubt that there are many - if any - religious members of the BHA or the NSS either.