Yeah, but there's vendors who do half assed Windows versions too - Oracle comes to mind. Ultimately the VB/.net shops like BMC do shitty Unix versions, the big iron companies do shitty Windows versions.
You got that right. In the end, the target platforms are the vendors prerogative.
Where you really run into problems is selling a weak Unix version into a Windows-hating shops. It ends up being more of a problem than if the vendor just told the customer "you're better off using Windows with our product".
Hate's a strong word, but yes, having a budget for such things in the past, a vendor coming in with an unpackaged app (or some horrible custom non RPM/deb packaging format), with no init scripts, no syslog support, and sales staff who don't understand the bog-standard RHEL or SLES platform to the point where you have to help them with their presentation, can and will guarantee no sale.
Where you really run into problems is selling a weak Unix version into a Windows-hating shops. It ends up being more of a problem than if the vendor just told the customer "you're better off using Windows with our product".