Indeed. Or decouple the ORM entirely, though that likely removes a key aspect of what Rails is about.
It is, however, what makes Ramaze so appealing. No coupling to any ORM, but there is a community inclination to use Sequel (so you can learn from example and there are people to answer questions).
there is a default to AR which is easy to overwrite with your ORM of choice (which often provides an ActiveModel interface and further rails integration by installing one extra gem).
tl;dr: providing sane defaults != strong coupling
The few times I've wanted to just start a Rails app with simple Erb, with models representing data concepts (but not necessarily SQL anything), and evolve out to using a database (if and when needed), have been frustrating.
It's something that has been easy to do in some other frameworks so I tend to see Rails as almost DB-required, if not in fact coupled.
If you're planning on using a DB eventually but don't want to deal with migrations etc. for now, then start with a NoSQL DB like MongoDB (you don't even need to describe your fields, it can be completely dynamic). You can always sub in an RDBMS later on if your design calls for it.
Works well, just don't require ActiveRecord and hey presto, your app doesn't have a database (or db rake tasks, etc).
Interesting that people do though. Any particular reason? What features of Rails did you still use? Seems like an odd use case.