At MatterHackers our customers are important to us, and we try to be as flexible as possible when it comes to quality control issues like the one you describe. It doesn't sound like you bought this filament from us, but if you ever do please keep in mind that we want to hear from you if there's ever a question of quality.
That said, 30 days seems a bit short for a warranty but I don't see anything underhanded or necessarily wrong with it. It's reasonable enough if the seller makes this absolutely clear and you agree to purchase the product.
I just thought of something else. With 3 mm filament, variances in the diameter of the filament have a much larger effect on the actual amount of plastic extruded. We've seen the diameter in different places on the same spool vary but up to 0.10 mm. This means that you could be pushing out 10% more filament (by volume) in some spots than in other spots. I wonder how much of the Z banding we see is actually caused by differences in the filament diameter.
i find with certain poor quality filament the moisture content is very high, this will steam and pop a lot, but if you do want to use it,i find if i slow down the print speed to 20% and heat up to 240 and walk away... tbh rather get some pla.