I mean, both use Intel HD Graphics 3000, and the MBP actually has a better processor. If they use the same Intel graphic card (and according to the website, the same info "13.3-inch LED-backlit glossy widescreen display", so they don't say they are different), how come MBP can't have 1440x900? What is the technical limitation of Apple hardware in MBP that justify this difference?
The simple answer is that the MBP doesn't have a screen that can display 1440x900 - it's native resolution is 1280x800 (ie. it only has that many pixels). This is a physical limitation, not a software limitation. There is no point in sending a higher resolution image to the screen, since it can't display any higher detail.
I can't tell you why Apple chose a higher resolution display for the 13" MBA than the 13" MBP, given that I wasn't party to those decisions. The reasoning may be purely historical, however. The white MacBooks had a resolution of 1280x800. Apple introduced an aluminum unibody version of the MacBook with the same display in 2008. By mid-2009, Apple decided to rebrand the unibody MacBook as the 13" MacBook Pro - still keeping the 1280x800 display. My guess is that they just haven't gotten around to changing it yet.
Your graphics card is capable of this and higher display resolutions, just not with the included LCD display. If you attach an external monitor you will notice it can go way higher, but obviously the hardware has to support this. The native resolution of your screen is a maximum of 1280x800, so it will not offer higher than than. It's worth noting that for the display to look crisp you should use a resolution that matches your native hardware, you can for example run a higher resolution and scale it down, but it will look bad, also you can run a higher resolution and have it scroll around so that it does not fit in your screen, but I wouldn't reccommmend this.