This is not inherently a problem.
Consider a much more mundane scenario, such as one might face in a police investigation: a series of events resulted in a result that is observable to all (for instance, the theft of a well-documented item, or a murder); the goal is to determine the nature of those events. Tools available include:
- Eyewitness testimony
- Direct observation of evidence left at the scene
- Knowledge gained from observing similar scenarios
From these, theories are developed that attempt to explain the result. These theories may align with the available evidence and prior knowledge, or they may dispute some of it as irrelevant or even misleading. Witnesses can be mistaken or duplicitous, evidence can be fabricated or contaminated, prior knowledge can be wrong... Any evidence must either support a given theory, contradict it, or be deemed irrelevant to it. This is not subjective. You may debate the validity of the theory or the evidence, but the relationship (if any) between the two should be established and can be documented.
...If that had been the nature of the question, then I would say it is reasonable - although not necessary on-topic here. If the question had constrained itself to actual, documented Islamic teachings, then it would have been able to fulfill both requirements.
But the question you reference did neither. It was both extremely broad (for which reason I closed it) and failed to actually be about Islam! No, "some unidentified people who claim to be followers of Islam believe X" is not a reasonable foundation on which to build a question - you're essentially asking askers to explain a vague claim that can (and will) change to fit the desires of those answering and/or the changing whims of the asker who can't be bothered to describe what he actually wants.
