Why are common criminals and political prisoners treated differently in the temporary lock-up?
I will add to the answer provided above. First of all, common prisoners are typically small-time criminals from the prole neighborhoods. The government in the novel does not seem to worry much about the prole criminals, as Winston describes them in Part 3, Chapter 1 as "drug-peddlers, thieves, bandits, black-marketeers, drunks, prostitutes." One example of a prole criminal who is all but ignored is the the heavily intoxicated and overweight woman who sits on Winston's lap. Winston also notes that the overall relationship between the guards and the common criminals was less intense than the highly abusive treatment that the "polits" received.
In addition, this treatment seemed to extend beyond the holding cell. Only the political prisoners seem to be taken to Room 101 while the common prisoners were allowed to curse and joke and offer bribes to the guards.
Finally, when in the labor camps, discrepancies continued to exist: "The positions of trust were given only to the common criminals, especially the gangsters and the murderers, who formed a sort of aristocracy. All the dirty jobs were done by the politicals." This treatment seems contrary to what one might expect and is because the political prisoners, Outer and Inner party member, are much more dangerous to Big Brother's regime.
In thenovel, only the political prisoners indicate what is perceived by the government to be a real threat. Because Big Brother has done away with any real sense of morality, common thievery is not really an important issue. Instead, threats to the way of thinking that is demanded by Big Brother are interpreted as dangerous. Because Big Brother operates by using brainwashing tactics to control society, the real threat to Big Brother lies in the possibility of someone being able to think freely and convince others to do so.