The reason behind drawing such a distinction may be more telling, as there is a larger push in some groups to draw this distinction for the sake of harkening back to pre-Islamic Persia. But of course, modern Persia and Islam are intertwined, so usually those who wish to draw a distinction do so for political or religious bias reasons.
That's a pretty big assumption. It's natural to like your heritage and it's fine doing so while being respectful of other people's culture and heritage. I like other people to know about my culture for what it really was, what it went through and what it is right now. This is in contrast with how western media has tried to depict Iran as their political foe. Interestingly, Modern Persia and Islam (more with Islamic rulers) are at odds right now, evidenced by recent social movement and political unrest.
You want to separate the scholarship. The problem is there really isn't enough historiographical work done here to meaningfully tease out the differences. Surviving primary sources often ended up in the hands of colonial governments which are loathe to open up access today or in the hands of estates of former colonial figures where they rot in a dark room somewhere.
Another large historiographic gap in the Islamic world around this time was the lifestyle of peasants. We have records of kings and lords because of the widespread practice of autobiographies. We also know the thoughts of philosophers based on their texts. But we lack a lot of knowledge about how peasants and other commoners lived around the time. If you think about it, that's the majority of the people living at any time.
