Why not ? Some people just don't care, and these who don't care don't care to check the option. So to me it's more logical (in order to have more data) to check the box by default.
Data collection is OK, but it must be opt-in only.
You're only looking at it from a "more data = better" perspective, instead of a privacy perspective, which is a lot more important.
We are talking here about anonymized diagnostic data, not private indentifiable information.
Additionally, "anonymized" data frequently turns out to be identifiable when enough of it is collected.
Canonical sent an email to a public list asking for feedback.
Your comment would stand if Ubuntu 18.04 was already out and this tracking was already in place, but it's not. It's being discussed.