Regardless of how products are manufactured or by whom or what kind of marketing campaign is used, the original question was why would someone feel compelled to trash a product they've never used.
Fundamentally, the reason why is because their sense of self is wrapped up in the
other product. Whether they're conscious of it or not (usually not), it has become part of "who they are" so-to-speak, or more accurately who they
think they are. If it is challenged or threatened, they themselves feel challenged and threatened and so must defend. Obviously, this is on a small-scale since we're talking about car sealant here, but the reactive pattern can be similar to even that of an actual physical threat, wherein adrenaline levels are raised, heart-rate goes up, etc.
The debate can go on endlessly and is not reserved for detailing gear, although obviously we here are sometimes really attuned to that. The PC vs Mac debate, as mentioned, also springs to mind.
Any consumer good, political affiliation, where we live, sports teams are fair game. It can also apply to car makes/performance (ahem

). The list is infinite and it all has to do with identifying oneself in part or whole with a product or some mental position and trying to "find" oneself in that and, in so doing, attempting to feel more complete. The problem is it doesn't work and never has. It may appear to work in the short-term by perhaps winning an argument or proving someone wrong or something (and that may feel good initially), but that is really just a band-aid covering up the real problem--identification with form.
In Buddhist culture (of which I know very little), there are physical forms (like sealant as a teeny-tiny example) and thought forms (like sealant A sucks). Excessive identification with
either is materialism and can be a cause of great suffering for that person even if he/she is unaware of it. You could even say that all of what I've stated above is a thought form, a position that I've identified with. And it would be if I derived my sense of self from it and felt the compulsion to defend it. I don't, so it's not. All they are are letters typed onto a computer screen--nothing more and nothing less.
To sum up, Setec had it in post 10 and Jakerooni's "ego-trip" reference is also true.