Because the Polish sentence either means that I don't read expensive magazines at all, or (a lot less probable) that I don't read two or more expensive magazines at the moment. One rather rarely reads two magazines at the same time, thus the first interpretation is clearly the best, although the second is possible. Anyway, plural is needed.
Just wondering, does "czasopism" come from "czas" and "pisać"? If so, it is actually similar to the Dutch word for magazine, "tijdschrift".
It sure seems so. Wiktionary says it's a calque from German Zeitschrift.
"pismo" is the whole notion of a writing system, it can be someone's handwriting, it can also mean some official letter.