As much as I try to believe it might be coincidence... No I'm rather sure they do it on purpose.
But a lot don't include batteries under warranty anyway or if so it's limited to 6 months or something. In a lot of cases lately I see that some even mention: the battery should work for 18 months or so after that it will start to decrease rapidly.
Another thing is battery tech it's not yet at a peak level I'd say. There just isn't incentive to improve it to much if it's at the "good enough" level for the companies.
I think the major problem is that companies tend to stuff so much into their devices that the tiny batteries they include just don't handle it well enough.
Take cell phones for example...
Companies try to sell so many extra things along with them. But what I would like to see is a low-feature phone with an eInk screen with some other bits included by default that would make it usable as a wifi AP for the 3g connection. And basic functionality. No extra apps and so on. Just something I can expect the battery to last for a fourthnight in standby.