Nobody has complained about SB including HW. That's a great deal.
But it would still be good to also allow people who want HW without having to get SB the option to still buy the first expansion by itself, just as we used to be able to. That's an ability that's been taken away unless you search around among third party retailers for one who still has old copies. It's generally a bad practice to take away options people used to have. Since SE already has standalone HW installations, it's not like it would take anything extra from them to continue selling them, and allow players the freedom we used to have.
The main reason for wanting one without the other is level caps. I've avoided registering HW all this time (despite the frustration about not being able to create an Au Ra alt) simply to be able to keep my level at 50 as long as it takes to go through everything the game has at that level. There is a ton of stuff to do at level 50. I've been level 50 for a little over two and a half years now and still haven't finished everything there is to do. There is no way I'd have been able to do nearly as much of it if I'd had Heavensward on my account. Even splitting content between different jobs, I couldn't have kept them at 50 nearly long enough. (And the relic weapon quests can't even be split between jobs, as it's a separate upgrade process for each of them.)
Once I finish all my ARR goals, I'm going to register HW in order to do it's content, and the same will happen at level 60. There's years worth of content at that level, and the only way people will get a chance to do it is by having a level cap that will allow them to stay level 60. That's only possible if we can register HW without registering SB. If new players trying to buy it now can't buy them individually, then most of HW's content will be lost to them. Despite Samurai and Red Mage being my favorite FF jobs, I'll gladly hold off on starting them in order to get the entire previous expansion actually playable rather than nerfed into a ridiculous joke by being 10 levels and a hundred or so item levels above it.