My plastic windows, which are about two years old, have water and dirt between the glass and frame! The window salesman told me this is normal because it is dry glazing. My question is, is this really normal? I've looked everywhere else and haven't seen this kind of accumulation of dirt and water inside the window anywhere else.
Almost all plastic windows have dry glazing. It is not normal for so much water to get in there. It is possible that the panel is not installed correctly. The water is more like condensation.
Putty was made in the days of single pane windows, that was a long time ago. Panels can also be made with silicones – probably along with foams, the most misused (and "for safety" preferably overdosed) substances in construction. If this were suitable for bonding, on the other hand, it would make non-destructive replacement more expensive and much less likely.
In this case, however, the problem is likely a combination of unclean manufacturing conditions and insufficiently sealed window panes.
Clearly the glass unit was not sealed. Try to call a technician and replace the seals. Maybe the corners are not mated properly.