The code is gratis. Although, partnership deals tend to go beyond simply sharing code, and into the realm of dedicating time and resources to working with each other.
We're talking about a curated, supported, official image here, right?
If folks want to use a "MyUbuntuImage" they or someone else packaged and uploaded, more power to them.
But by pulling a Canonical image, you have a relationship with Canonical. Expecting that relationship not to exist "because open source" seems to be misunderstanding who does what work.
As to whether this should be opt-in, done, etc. is another matter entirely. But the fact that it exists at all doesn't feel particular shocking.
It's not like we're talking about everyone who pulls a RedHat image's info being sent to Canonical!
I don't expect an OS based on an OS based on an OS based on a half-finished OS based on free software principles to have shady data-dealing attached, yet hidden from the people whose data is being dealt.
I mean, it's kind of ridiculous to think that you could do anything in a cloud environment system and not have your actions tracked. Hell, with automated load balancing and load-based billing, that's literally what you're signing up for.
Privacy protection is not an obligation, but transparency and openness is. Yes, you're not contractually required to not make a separate computer system that's proprietary and closed and disempowering, but that's so pedantic as to be malicious.