Walls of oval pool starting to bow out

We just had a 15x30 AG pool put in about a month and a half ago. It's made by "USA Industries" and looks like the one at the bottom of this page:

http://www.usa-industries.com/pools.html

It has six buttresses along each side that hold the walls vertical. However, I noticed that both walls starting to bow out. In the middle, the top of the wall is out about 1.5 or 2 inches as compared with the bottom. The two curved sections are fine.

The pool company said that they could fix the problem, but they want half the water drained. So, I can either wait until fall and do this, or try to fix it myself. Is it a good idea to use a few ratcheting tie-down straps to try to pull the sides together?

I noticed that the pool installers used concrete blocks under each of the vertical wall supports, but not under the buttresses. I wonder why.

Judging from the picture you linked to, your pool looks to be made from paper and pencil crayon, this could be a problem!
I'd have to say that your installer screwed up by not putting blocks under the buttresses since that's one place you will get a lot of pressure from the top of the wall moving outwards.
All walls will move outwards eventually on an oval pool, it's the nature of the beast when you buy one, on a round pool they won't because pressure is equal in every direction.
I would not try to ratchet the pool back together, you can cause damage through an upright buckling, that will take your wall with it an collapse your pool, do what they suggested and drain half the pool to alleviate a lot of extra weight when doing it - this is likely their plan as well, once it's pulled in they or you will place patio blocks under the uprights - it should last at least one season, once winter frost has it's way with the pool, it'll lean outward again - maybe not as much to start but it WILL lean out.

So, would a buttress-free oval pool have been a better choice, or do they have other problems?

same issues, it's the nature of the beast with oval, there are different forces acting on it in comparison to a round pool - read my explanation of them at:
http://www.poolforum.com/pf2/showthread.php?t=7665