|In a perfect world or company, if you are falsely accused, you get an apology, and it's perfectly normal to expect one in this situation. Even if the senior managers were not up to apologizing, a brief acknowledgement would have helped.|
However, looking at the matter from their side of the desk, they thought an error was made, they sent an email, and they heard back that all was well. From that point, the issue was resolved and they probably focused on more pressing matters, believing that no further communication was warranted. It is possible that they are treated this way by their own managers, so they believe that this approach is normal and acceptable.
In addition, it is very easy for this type of problem to occur when communication is through email. You may have read a more negative tone into their email than they intended. Perhaps you would have had a more productive exchange, including an apology, if you had responded either by phone or in person. That is something to think about the next time they launch one of their critical emails.
At this point, if you ask them for that apology, you are going to appear overly needy, and you don't need that.