Yeah, sadly that's what I'd expect: the corporations like to keep the media you paid for in a place where they control them rather than you. I don't suppose Google allow download of stuff that you uploaded in the first place (i.e. download the file rather than download to Google's app)? It may be a long shot, but I'd make sure there isn't something in the settings, or a way of doing it from their website.
To be honest, epubs don't take a lot of space so I'd personally have kept a copy anyway. One of my principles with ebooks is that if I paid for them I want a copy I control, which I can load and read on any device I want, rather than letting some corporation (Amazon, Google, Kobo, whoever) control my access to my books.