Because (the familiar is a spirit conojured by the welock) the familiar is completely subservient to the Warlock.
Agreed. But this doesn't mean that it lacks free will. It just means that the Warlocks will can override the Familiars. A command to 'do X' contains a lot of leeway on
how X is to be achieved.
Example: A Warlock orders its Imp to 'Kill that Orc!'. If the Imp lacked free will, it couldn't act on those orders. We instead presume the Imp has enough free will to decide
how best to act on those orders (subject to its own memory, logic, reasoning and experience).
It could poison the Orc. It could cause an avalanche on the Orc. It could try and rip the Orcs throat out. It could try and persuade the party barbarian to stab the Orc. It could use any of a nigh infinite methods to do what it is commanded to do. Its end goal is only (one dead Orc). Presumably the order is 'Kill the Orc as quickly as possible', but bear in mind the order itself didnt set a time frame for completion.
The warlock can command the Imp familiar to do X, but the familiar (being Evilly aligned) can subvert those orders (a LE Imp could choose to do so by following those orders literally, and against what the Warlock actually intended).
Consider a Warlock who has a rival in town, who has just beaten him to the position of Town Mayor (the Warlock is the deputy and if the Mayor dies, he gets the title). The rival is also a former lover. The Warlock wants her to be happy for her whole life despite this rivalry, and (rather casually) after the election, orders his Imp to 'Take care of her'.
What does the Familiar do in this case? It could ask for clarification on those orders (take care of her... or
take care of her?). It is under no compulsion to seek clarification from the Warlock, and may kill her (despite the Warlock just wanting her looked after and pampered) or vice versa.
It can't disobey them, it can't mislead them, it can't bristle at being mistreated.
Source? The text only tells me it cant disobey him. It doesnt tell me the familiar knows the mind of the Warlock and knows his intent, or that it isnt free to follow those orders subject to a certain level of of initiative.
The rules definitely say nothing about not complaining about its service (it might follow them quite grudgingly complaining as it does so), or even possibly misleading the Warlock (aside from in the absence of some very stringent orders not to - orders that cant be interpreted differently)
The Warlocks familiar is a gift from his (usually dark) master remember. His master may also have an agenda at work here - corrupting the warlock further, or even getting the Warlock killed to claim his soul.
Im not saying here that one should us the Warlocks class feature to troll him and kill his PC (although a plot involving conflict between the Warlocks patron and the Warlock involving the familiar could be quite interesting if done correctly and not as a 'screw you warlock' thing that strips class features from him arbitrarily) nor am I saying that a DM should run the Familiar like an antagonist or make it more of a pain in the backside than a useful class feature.
Only that a (reasonable) interpretation of the class feature is that it (in effect) creates a loyal and obedient (but evil) NPC as a gift from (a probably even more evil) master. Within the scope of that definition, many twists are possible with a bit of imagination.
At a minimum the Warlocks patron could decide to grant the Imp a modicum of free will, or tire of having to constantly replace it (the spirit itself might resent the Warlock for getting it killed all the time - the familiar dying might have consequences for it in its own home plane or other 'metaphysical' consequences).
Im of the view that the DM should play the familiar as a (loyal but evil) NPC, complete with a personalty of its own. The Warlock player plays the Warlock; its the DMs responsibility to play everyone else (barring other PCs of course). I use the same guideline with Simulacrums and summoned creatures, created undead and so forth. I might let the Warlock run them from time to time to save me time, but I'll often simply have the warlock issue orders to the summoned/ created creatures (or familiar, or undead or whatever) and then interpret those orders (as the creature) and go from there, taking actions the creature thinks are appropriate under those orders (and within their parameters, as interpreted by the creature).