“δουλους” could mean “slaves” or “servants” depending on context (Thayer's Greek-English Lexicon). In the context of a Christian's relationship towards God, “servants” seems appropriate since believers become δουλους by acceptance. The voluntary aspect indicates servanthood rather than slavehood. The NASB also translates “δουλους” as “servants” in Revelation 10:7. The ESV 2011 Update has changed "slave" to "bondservant" in 1 Corinthians 7:21 and elsewhere (Link to the List of Changes). The following explanation of the move from "slave" to "bondservant" is explained by the ESV translators in the new Preface (Link to the Preface):
Third, a particular difficulty is presented when words in biblical Hebrew and Greek refer to ancient practices and institutions that do not correspond directly to those in the modern world. Such is the case in the translation of ‘ebed (Hebrew) and doulos (Greek), terms which are often rendered “slave.” These terms, however, actually cover a range of relationships that require a range of renderings—either “slave,” “bondservant,” or “servant”—depending on the context. Further, the word “slave” currently carries associations with the often brutal and dehumanizing institution of slavery in nineteenth-century America. For this reason, the ESV translation of the words ‘ebed and doulos has been undertaken with particular attention to their meaning in each specific context. Thus in Old Testament times, one might enter slavery either voluntarily (e.g., to escape poverty or to pay off a debt) or involuntarily (e.g., by birth, by being captured in battle, or by judicial sentence). Protection for all in servitude in ancient Israel was provided by the Mosaic Law. In New Testament times, a doulos is often best described as a “bondservant”—that is, as someone bound to serve his master for a specific (usually lengthy) period of time, but also as someone who might nevertheless own property, achieve social advancement, and even be released or purchase his freedom. The ESV usage thus seeks to express the nuance of meaning in each context. Where absolute ownership by a master is in view (as in Romans 6), “slave” is used; where a more limited form of servitude is in view, “bondservant” is used (as in 1 Corinthians 7:21–24); where the context indicates a wide range of freedom (as in John 4:51), “servant” is preferred. Footnotes are generally provided to identify the Hebrew or Greek and the range of meaning that these terms may carry in each case.
The ESV translators do not agree with the KJV translators in every instance of translating "δουλους", but they nonetheless recognize that the context determines what the word should be in English. It is an error to say that all translations of "δουλους" as "servants" is wrong. As stated earlier, the context of Christian soteriology justifies translating "δουλους" as "servants" in almost every instance.