This is debatable, as written.
Er... no it's not. They are explicitly allowed to choose from the normal list at any point where they are given a power of a listed level.
The final arbiter comes down to this, if the Warpriest is NOT forced to accept his Domain powers as fixed power selections, then why the h-e-double-hockeysticks does the class exist? Its a Cleric with a couple of preselected rituals instead of Ritual Caster. Its either a blank slate filled by one choice, or its an entire waste of design resources to build the same class with another name.
How many design resources were wasted? The new powers are also usable by the cleric. It let them provide the cleric in the Essentials book that used 95% of the same design of the standard cleric, trading out ritual casters for, as you note, some preselected rituals, and a bunch of 'default' choices. That seems pretty much perfect as an offering for new players.
The problem with the Slayer(specifically) is:
1. It doesnt accomplish anything that couldnt have been done with a 4e Fighter or Barbarian.
What would you have done to alter the 4E fighter or barbarian to provide the same benefits as the Slayer?
Remember, folks like the stances and power strike and find them simpler than the at-will/encounter/daily system. I'm confident you could have built a customized Fighter that you found easier to use than a Slayer - but clearly not one that would have helped me, or the various players I've seen who have concerns when dealing with the power system.
2. It takes/took up design time/space that could have been spent adding these options to base 4e.
It's absolutely true. But many folks didn't like Psionics, either - did that mean they shouldn't have produced it, since that took design space away from supporting existing options?
By this logic, they should have put out the PHB and just supported those classes, never adding anything new at all. No thanks.
Many folks like the Slayer, and it very directly answers concerns they had with the game. I get that you don't like it, but you seem to be going out of your way to try and insist that those WotC should not try and produce material for those folks, just because you don't like it.
3. It is being expanded upon to the detriment of those base 4e classes.
... I'm not sure I've seen this. We've had all of one PC book come out since Essentials, right? (Which hasn't supported the Slayer at all). It did support some Essentials classes... but, honestly, offered as much (or more) support for the PHB Cleric and Wizard.
I'd be absolutely fine with some more support for the Slayer, myself - though I don't think it needs it. What I don't want to see is more support for the Fighter or Wizard, who have buckets and buckets of feats, powers, etc. Yes, I'd like more material for some less-supported classes, but I don't think you can blame Essentials alone for that. Yes, I'd like to see WotC ramp up the content again in general - but, again, that hardly is something somehow caused by the Slayer.
By definition, Essentials should be a dead line. If these are supposed to be the new entry points to teach people 4e or they are there for those people who want a simpler play experience then expanding on them is anti-thetical to their design. If you designed a class to be simple to play, adding newer and more complex options to it instead of directing people who want more complexity to the parent class is idiocy.
Well, again, it is an issue of the system. I think absolutely the ideal would have been to rebuild the system from the ground up so that transition is a smooth one. But that would have been even more disruptive.
As it is... those 'parent classes' exist. WotC can totally say, "Hey, want a more complex Knight? Check out the PHB! Or DDI! Or this free article on our website!"
But what is wrong with them producing new options that can support the new material and the old? For example, for those who like a middle ground? Or for those who like the Essentials classes or structure, and are happy to see more options with them, without expanding to the option inundanation of a Fighter with hundreds and hundreds of feats and powers?