This is why you do an internet search instead of just looking at what the vendor says, and/or look at what the vendor actually ships for support. If you see something like a binary blob, run away. If you see stuff shipped upstream, go ahead.
As a counterpoint to your experience, I've had great luck buying laptops off of Canonical's approved hardware list.
As a corollary, I've had worse luck buying computers that come with Linux pre-installed, only to discover that the pre-installed distribution is very specifically and carefully configured and includes proprietary bits and it's impossible to install any other Linux on the device or upgrade it.
As a counterpoint to your experience, I've had great luck buying laptops off of Canonical's approved hardware list.
As a corollary, I've had worse luck buying computers that come with Linux pre-installed, only to discover that the pre-installed distribution is very specifically and carefully configured and includes proprietary bits and it's impossible to install any other Linux on the device or upgrade it.