You get domain spell slots as a cleric. You could choose no domains a level 1 cleric and still get domain slots -- you couldn't use your domain slots, but they'd be there.
Who cares if they exist or not if they're absolutely worthless? Picking Law Domain over Law Devotion means you have 1 extra spell of each level each day, and the low level Law Domain spells are not bad spells (in fact, Magic Circle Against Chaos and Shield of Law are two extremely solid spells, and Dictum has a lot of abuse potential). This far outweighs the value of ANY feat, including Law Devotion. What's more, you can take a level of Ordained Champion to lose the crappy Law Domain ability and get a Fighter feat, which makes up the opportunity cost of not getting Law Devotion by sacking your Domain.
As such, in this specific case, not sacking Law Domain for Law Devotion is absolutely the superior choice. Note that I'm not saying to not take Law Devotion ever, just don't take it by sacking a domain.