The Watchtower calls everyone who leaves their organization an 'apostate'. 
Jehovah's Witnesses believe that anyone that leaves their religion and disagrees with the Watchtower Society is 'slandering' God's name. Their literature says that apostates speak half truths, twisted things, and their words are 'valueless'. 
So it's easy to understand why Jehovah's Witnesses reject all opinions that are contrary to the teachings of the Governing Body of Jehovah's Witnesses.
The organization says that it is the only true religion. All other religions are 'false' and part of Satan's organization. These will be destroyed by Jehovah at armageddon. Only Jehovah's Witnesses have any scriptural hope of surviving. 
Can these claims be proven using the Bible? Are these beliefs Bible facts or Bible based opinions? What Bible scriptures can be used to prove Jehovah's Witnesses are God's organization? If you can't find any, ask yourself if it's religious propaganda. See Mind Control
Jehovah's Witnesses do not own God's name and he hasn't give them his name to use exclusively. Anyone can use it. It's an ancient name. Just because someone starts using the name Jehovah, doesn't mean God approves of everything they say.
Skeptics say the belief that only Jehovah's Witnesses have the truth and represent God because the word 'Jehovah' is in their name, is an opinion, assumption and more religious propaganda. It's not a fact.
The Jehovah's Witnesses believe that Jesus appointed a 'faithful and discreet slave' to represent him in his absence. They believe they must obey God's slave. In the early days, God's slave was thought to be Pastor Russell, their founder. Then it was Christian ministers, ... then the 'anointed remnant', and now the Governing Body. Is it true that Jesus requires Christians to believe everything the leadership of the Watchtower says? Where is this written in the Bible?
Critics say the Bible and Jesus are silent about the identity of the faithful/unfaithful slave or servant. Read what Jesus said about the slave parable in Matthew 24:45-51 and pray to him for understanding. See What The Bible Says About the Faithful and Discreet Slave.
Claiming to be God's slave could be viewed by God as blasphemy against the Holy Spirit. See Salvation. The Best Religion. Can It Save You?
Not only that but adding to God's word and running ahead of what Jesus said might put the salvation of all Jehovah's Witnesses in question. The Bible says, 'Anyone who runs ahead and does not continue in the teaching of Christ does not have God; ... 2 John 1:9
Jehovah's Witnesses assume that the one and only true God, the Father of Jesus Christ is Jehovah. Critics say that Jehovah is a very different God. Examine the evidence in this article, Jehovah vs Jesus?
Once someone understands that the beliefs and claims of the Watchtower belong to them and not to Jehovah, any reasonable person can see that disagreeing with the Watchtower's Bible based opinions, is not slandering Jehovah's name. It's not slandering the faith of Jehovah's Witnesses either. All religions including the Jehovah's Witnesses are entitled to opinions and beliefs.
All Christians are searching for truth. The only way to do that is by entertaining different ideas and agreeing to disagree. Christian growth and a greater understanding of God's purpose is impossible when disagreement with a man made publishing company is considered rebellion against God. 
1. Apostates speak lies, half truths, and twisted things. Who are they and how we can protect ourselves from their deceit? See Lies
Why does the Watchtower use the word 'apostate' to label everyone who leaves their organization? No one can be sure. However according to the Watchtower, name-calling can be used to create prejudice against entire groups of people so that 'people reject the person or the idea on the basis of the negative label instead of weighing the evidence for themselves.' From the Awake! Magazine June 22, 2000 pages 6 - 10. See Why Do Jehovah's Witnesses Fear Apostates
3. "Rebellion against the slave is rebellion against God." Watchtower June 1, 1956 page 346 par 11. See Organized To Do Jehovah's Will
4.' The slave deserves our complete trust.' (Watchtower Feb. 15, 2009 Page 27) See The Watchtower Organization Says It's The Faithful and Discreet Slave, Watchtower Feb.. 15, 2009