It's a matter of purity inference. They already trivially know the value isn't used based on the AST, so they need to show that the function being called is pure to elide it. Naturally, in the general case this is undecidable, but there are fast algorithms for very weak purity inference that might do the trick, at least in this case.

There may be other Python-specific concerns I'm missing - my work in this area is in Ruby static analysis - but one other thing is that allocating memory is a side-effect itself. The main side-effect visible to Python is that it might raise an exception for being out of memory - this would have to be special-cased as an acceptably ignored side-effect by any purity analyzer.