“The jar is filled with sand” is not really the kind of sentence they object to (or should be objecting to). Rather, it’s when the passive voice makes the sentence awkward and unwieldy (“I was told by him to shut the door” rather than “He told me to shut the door”) or, worse when it’s used in a cowardly manner to avoid naming names (“mistakes were made”). But there are lots of valid reasons to use the passive voice — especially when the object of the sentence should be emphasized over the subject. (Notice above that I just used the passive voice — “when it’s used in a cowardly manner” — because it’s the word, not the user, that’s more important to our discussion.)
“The jar is filled with sand” is perfectly fine. “Sand fills the jar” would be a little bit odd, in fact, unless you’re talking about the action of filling up the jar, rather than the state of being filled with sand.