Here's the thing... at least here in the U.S. when I pickle onions, the liquid that's poured over them (vinegar, etc.) is hot and then the jar(s) require immersion in a boiling water bath for about 20 minutes or so. When complete the special lids used for canning have a small nipple-like raised area in the center so that, as the jars cool, the outside airpressure "pings" this raised area downwards to announce that the jar has sealed correctly. With a plastic jar, the sides would more than likely collapse inwards due to the same airpressure differential and not allow a good seal. This could lead to botulism or mold... not a good thing, no?