I think this is a problem with perspective. Linux on the desktop is not a homogeneous collection controlled by a single entity. It is a collection of individual pieces. Software vendors have little incentive to make their software work 100% with those individual pieces on the Linux desktop, unlike they do on platforms where they sell other software and services. You get what you pay for, on the Linux desktop, your payment will be in time and effort to get the (usually substandard) free offerings from these software vendors to work properly.
It does seem that this is one of the problems, though surely not the only problem.
If you're a hardware vendor making a new USB device, you're going to alter your device and your drivers (if you have them) so that the item works well under Windows and maybe OS X. You're probably not going to try to tweak it to make it work with the Linux USB drivers.
Linux is in this odd position of having to try to copy all the functionality and interface of the Windows systems because that's what all the hardware has been written for. It makes for a buggy and error prone process.
I don't agree that you get what you pay for though. At this point it's all about market penetration. If Linux was 90% of the PC market, don't you think those USB webcams would be likely to work the first time? Don't you think that if a company wanted to enable new functionality they'd submit a driver to the kernel? It wouldn't take any more effort from them (less if you consider programming for Linux easier than programming for Windows) and the Linux users would get the benefit.
Let's branch into another universe where everything is exactly the same as this one, except Linux (in the same state it's in now) is 90% of the PC market. But how can such a world exist except by ignoring the reasons that exist in our universe why Linux is not the OS for 90% of the PC market.
It's not a sensible "if". If Linux was forced onto 90% of computers early on, it would have been forced to be a lot more stable, backwards compatible and rigid than it is now. The webcams would work but the OS wouldn't be the Linux you know today.
