The fact of the matter is there are proprietary drives in Ubuntu and you choose whether to use them or not, why should this service be any different?
I think the difference is that this is proprietary software (or at least the back end is) that is branded as Ubuntu. Personally I don’t have any problem with Canonical making money from this commercial service, just as I don’t have a problem with them selling Ubuntu t-shirts and mugs. My annoyance is Canonical using the Ubuntu branding to market a closed source program, when closed source programs are against the Ubuntu philosophy
At the core of the Ubuntu Philosophy are these core philosophical ideals: 1. Every computer user should have the freedom to download, run, copy, distribute, study, share, change and improve their software for any purpose, without paying licensing fees. Our philosophy is reflected in the software we produce and included in our distribution. As a result, the licensing terms of the software we distribute are measured against our philosophy, using the Ubuntu License Policy. .... we are working to ensure that every single piece of software you need is available under a license that gives you those freedoms.