Complex sentence: Some of these were straight clones, simply intended to be direct substitutes and taking advantage of the lack of copyright protection in the phototypesetting font market of the 1960s and 70s onwards.
Split into simpler sentences: Some of these were straight clones, simply intended to be direct substitutes. This profusion of pirate copies took advantage of the lack of copyright protection in the phototypesetting font market of the 1960s and 70s onwards.