I am not sure what type of windows they are (are they double hung?) but I am guessing they are saying this because they have had issues with people damaging the windows with air conditioner units or they do not want to be held liable for units that fall out of a window.
I have never seen a window "Designed" to hold an AC unit they are designed to be good windows and nothing else. Just because you ac unit fits in the window perfectly doesn't mean they designed it for that reason (if you get my drift)
I believe they are saying the same thing as "add an AC unit at your own risk" The window won't be falling out of your house and you really can't break the glass because the glass is above the unit. The only think you could damage is the window frame where the unit rests.
However, some windows simply can't hold a unit so it depends on the type and style of window you are getting also.
-renee

The key word is "support". Windows are not designed to support an A/C unit. A wall mounted bracket and shelf should support the unit so that no weight is on the bottom sash.

I've been on service calls for some window companies and you wouldn't believe some of the haphazard designs some homeowners have come up with. Some were using the sash to support the total weight of the A/C unit. Not good!

If you mount the A/C unit according to unit directions, the window won't be supporting it.

Make sure that no condensation leaks on the sill. That will cause water damage and eventual rotting the sill and jambs.

Michael

The Pella contract is specific on the fact that you are liable for any damage created from an AC unit. There are a lot of companies that offer special sills that are made to support a window AC unit. I know Schuco offers them and also offer a small sash that goes in so you can lock it down to the AC unit. Kind of a mini sash to make your window look normal. I would ask your salesman if they offer a special sill for the AC window. See what he comes up with. Post back when you find out.