Let me put it this way: I can make infinite creatively worded claims, which can in no manner be disproved by you.
None of which would be falsifiable (c.f. Karl Poppler) by experiment, and therefore not scientific in nature. Hence the fallacy of demanding proof and referring to scientific methods in the middle of the discussion.
The irritation is that we have a casual conversation. At no point (and the point for this would be the very beginning) is it declared to be a scientific discussion. No one agreed to adhere to rigorous scientific methods. But then you say something that the other party happens not to like, and suddenly an outcry is heard for scientific proof.
Like I said, it's just a way to demand that the other guy substantiate his beliefs while feeling no obligation to return the favor. It's a desire to put the other person at a disadvantage while maintaining one's own air of legitimacy. It's that instant "hah I win, you lose" effortless slam-dunk victory everyone on this site seems so desperate to experience. Personally if I want to outsmart or out-debate someone I like to have worked for it, feel that I have earned it, and most importantly I make my intention known from the start. If I have to use cheap-shot tactics to avoid a level playing field, just to feel like I am right, then my own views must not be very sound.
I have speculated in the past that the constant reoccurrance of this phenomenon is likely because of personal insecurities on the part of many Slashdotters. It matches strongly the need for validation and a sense of superiority that insecure people tend to display.