No, it doesn't.
Marco has been talking a lot on his podcast about dropping support for the iPad 1 for Instapaper as well in order to use some of the new iOS6 features.
The 3GS effectively has better hardware than the iPad 1.
CPU, RAM, etc. are basically the same, but the iPad has to drive about five times as many pixels. Thus it effectively has much less RAM, an underpowered CPU/GPU, etc.
It's an odd and unintuitive situation, but despite being newer than the 3GS, the original iPad is ultimately a less powerful device, when it comes to which apps and OS releases it can support.
I'm sure Apple could support iOS 6 if they wanted to, but I imagine they decided that it wasn't worth the tradeoffs (e.g. poor performance, RAM available to apps, resulting stability or lack thereof).
They were still selling the 3GS until just a few weeks ago; it would have annoyed many customers if iOS 6 wasn't released for the device. The first-gen iPad, on the other hand, stopped being sold when the iPad 2 was released in March 2011.
iOS 5 also doesn't run all that great on the iPad, so I'm sure that played into the decision not to offer iOS 6 as well. The 256MB of RAM paired with the large screen seems to be the biggest cause for this. The 3GS' hardware isn't great, but it's only powering a 320*480 display