The attitude is derived from a general animus many have towards AI companies. They resent the efficacy of AI because it devalues individual expertise.
I can't imagine it really justifiable to say that training off data is the same as "stealing", when that same claim, that learned information that a person could retain and reproduce constitutes copyright infringement is the subject of many dystopian narratives, like this one, where once your brain is uploaded to the cloud you have to pay royalties based on every media product you remember.
Part of how AI works is that it's just really complicated compression, you can get AI to write out Harry Potter novels word for word with the right prompting.
When it picks out a rare bit of code, it will be simply copying that code, illegally, and presenting it without attribution or any licenses which is in fact breaking the law but AI companies are too important for the law to apply to them.
There's been instances where models have spat out comments in code that mention original authors, etc., effectively outing itself as a copyright thief.
There's nothing anyone can do about it, but the suspicion is that the big companies have taken everyone's code on GitHub, without consent, and trained on it.
And now are spitting out big chunks of copyrighted code and presented it as somehow transformed even though all they've actually done is change a few variable names.
It is copyright theft, but because programmers are little people, not Disney, we don't have any recourse.
This is the answer. People don't like having their livelihood threatened so they kick the thing that threatens it.