I get notified of great offers from GN and a couple others ie: LVH and I haven't gambled there in 5 years. I would not know of the offers if I didn't sign up for their email. So I have no idea why you disagree with this as it costs nothing but a minute of time.
IF it is just giving them an email addy, I see no issues nor disagree.
I'm just pointing out that for non club members, if you have to "register", than you become a member of the players club without receiving the sign up bonus, etc when you do go there.
I'm not saying it will happen that way, but that it may have that result.
OTOH, I believe it was Venetian - Palazzo that gave an extra $5.00 or $15.00 a few years ago for joining their players club online. Still had to show ID, to get a card and FSP.
I'm just cautioning that there may be a reason to check with each place before "signing up" online for their rules, etc as far as new player bonuses.