MechaTarrasque
Hero
I think because they want to downplay the closeness of the relationship between the Warlock and their patron as opposed to the Cleric and their deity. The default flavor of the Warlock seems to be that although you got your powers by associating with fiends/lovecraftian horrors/fair folk, that you're still a somewhat heroic character and not a minion that's actually advancing those beings' agendas (with the other players as convenient pawns). Yes, you can certainly play a Warlock character like that, but I don't think they wanted to encourage it.
Compare this to Clerics, where they seem to have fewer issues with the idea that your character is furthering their deity's cause and as such can call on their deity to intervene and send them (somewhat) regular support. Keep in mind that while one could theoretically summon a fiend, that the deity's PCs have relationships with tend towards the Good/Neutral end of the spectrum.
That is a good thought. Planar Ally would definitely give the patron the power to punish a warlock: use a 6th level spell (the only one a warlock will cast that day) and get a dretch or a mephit is a pretty big hint that the "boss" is annoyed with you. Of course, if you were planning on playing a warlock who was on the outs with his/her patron, you probably wouldn't pick that spell.