I think a lot of the time this is because people aren't careful about thinking through their additional criticisms and rely on their general feeling of "I'm upset with X and this appears bad on the surface" to suffice. When people call out the obvious faults with the argument, people take it as defending the company rather than pointing out faulty assumptions (probably both).
A simple example of this is 'Google abandons products.' it's been more and less teue at different times, but it's almost always presented as a truism when the reality is more nuanced, and depends on a bunch of factors (is it a paid product, is it out of beta, do they publish a support lifetime, etc.) Google do seem to abandon a lot of stuff, but they also have a lot of stuff which can skew perceptions if you are mostly aware of certain products, and they seem to follow a sort of consistency as to how likely they are to persist.
An overly broad assertion about Google abandoning products may be met with responses like thus, even if the people responding agree that Google does it more than others. That's not defending Google because it's Google, it's adding nuance to a discussion (there are of course just defenders).
Note: I'm not trying to argue this Google criticism, it's used as an example. Even if you think this is a bad example because that's not how you see this argument usually played out, it's probably more constructive to discuss communication strategies and misunderstandings than to rehash a very played out Google criticism here more than I've done.