re: Teedy apparently took away the cannon on the riverfront at Jackson Square yesterday

That looks like / I think that is a Parrott rifle. If so, that was a type used mainly to almost exclusively during the Civil War. So notwithstanding that AFAIK the Washington Artillery saw action in other wars, that's probably why the cannon, and maybe that memorial or whatever, is being removed.

[ETA]
Not sure why the downvoting--I am just providing factual information that may be informative to some.