I don't see this as inappropriate at all. If someone legitimately has that seat, all the things I pile up in that seat will not (and shouldn't) keep them from their seat. If that seat is NOT their assigned seat, then they don't have a right to it anyway (and as someone else mentioned, an elite flyer might have some right to having that seat empty). Given that FA's do not always enforce these matters, seems reasonabel enough to take small measures like this.