I suppose at least part of the philosophical issue here is that conservatives tend to believe that the federal government shouldn't be doing anything that's "non-essential" in the first place. And yeah, from there it gets progressively more contentious.
Which is ironic because in theory the military and veterans programs are like 60% of the "non-essential" spending.
How do you figure that? Here's where the scope of legislative power
(and thus, legislative funding) is defined. Note, that there are 6 different items listed which touch on aspects of maintaining a military, from dealing with piracy on the seas, to declaring wars, to funding an army, and a navy, a militia, etc.
No mention of funding healthcare, or providing assistance for the poor, or housing for the homeless, or education funding though. So explain to me how you are deciding what is essential and what is non-essential?
That's one of those distinctions that seems rather arbitrary at times (the definition of "discretionary" and "non-discretionary"). I don't have anything against the theory of setting budget priorities and whatnot in theory, but these seem like manufactured crises to try to force issues that are super important to a political base ("the wall", DACA, etc with the current round), but that the general public isn't really overly concerned with.
Ironically, most non-discretionary funding is for things that aren't actually listed in article 1 section 8 of the constitution. One might suspect that is specifically because in the absence of special legislation mandating such funding, it would likely not otherwise be funded (cause it's not actually in scope). This is the result of decades of significant expansion of government funding over time though, and there's a whole historical aspect to this.
As to the point you were making though, you're correct. In this case, it's pure politics. There's nothing specifically budgetary about the issues being fought over here. Immigration reform is a subject all its own, and should be treated that way. There's no reason why passage of a budget bill should be held hostage to issues on immigration.
Now, in the context of immigration itself, it's perfectly legitimate to compromise. So one side wants DACA recipients to get permanent visa status, then they have to give something up. And, honestly, in this case, it's not even an arbitrary "give us something unrelated that we want" deal. There's a certain logic to balancing out granting permanent visa status to folks who are here because their parents brought them through our porous border security as children with some sort of effort to tighten that security up so that we don't have the same problem in 10 or 15 years. If you had a bad alignment on your car, and it caused your tires to wear unevenly, you'd certainly see the need to replace the tires that are about to blow out, right? But you'd be foolish not to also fix the alignment so that you aren't right back with pre-maturely worn out tires in a few months, right?
You fix the problem that created the condition as part of treating the condition itself. Doing otherwise is foolish. If we all agree (and I hope we do) that the current condition of DACA recipients is harmful to them (stuck in an illegal status, through no fault of their own, and often with no ties to their nation of origin), then we should all agree to work to prevent the same condition happening to more people in the future. What's strange is that the Dems don't seem to want to do this. It's almost like they want to make sure there is a steady supply of victims of our broken immigration system available for them to trot out and use for their own political agenda. But that would just be crazy talk, right? Right?