I don't necessarily think that they are trying to sell you the "more is better" song and dance. I think what most people are saying is that 300mgs will shut you down just like 500mgs, 750mgs, hell...5000mgs. It is exogenous test and will shut down your endogenous testosterone production and therefore require a PCT to gain normalcy.
With that in mind, if you are going to shutdown, you might as well try to maximize your gain to side effect ratio. I don't think you would find anyone on here that would recommend 5000mgs per week because the side effects and costs would outweigh the gains that you would see. But the flip side of the coin would be that 300mgs would probably not net yield enough gain to warrant complete shutdown.
I don't want to speak for everyone, but usually, that is my logic when I recommend against low dose cycles.