While it is true that you can damage or unduly fatigue a spring by forcing it in a direction that inhibits its ability to "spring back" into shape (pulling it apart or over-compression), any properly designed and manufactured magazine spring will experience fatigue in normal use through compression and decompression cycles. Problems with set and creep are mostly poor design or manufacturing quality control and, yes, selection of unsuitable materials based on cost. All materials have a fatigue limit and spring steels are no exception.
Springs are typically made from cheap high carbon or stainless steels, and the properties of those base materials are not particularly well suited for use in springs that will encounter a large number of cycles or high loadings. Chrome silicon steel stock is a more suitable material for springs that will experience a large number of cycles or high loading.
Flat wire springs tend to work a little better than round wire springs because they are able to sustain greater compression and the shape of the material is more suitable for applications with uni-directional loading similar to what a buffer spring would experience.
That's my layman's understanding of how such things work based on an explanation from someone who knows. If you want to know more, contact a mechanical engineer, metallurgist, and people who make springs. There are plenty of excellent explanations, although most are quite technical, about the failure modes for springs and spring materials.