You really don't want anyone to use any of the premixed grouts. There are many horror stories about most of that stuff. Standard everyday Portland cement grouts have been around for decades and have served the industry well if installed properly.
Here's some things to look for.
(You don't mention how much tile you are talking about and the quantity would be good to know.)
1. ALL grout manufacturers recommend that the installer dry-mix the entire container of grout powder before adding any water. This will eliminate any settling that may have occurred during transit and storage. In the event more than one container of grout is necessary then the contents of all the containers are to be interspersed while in their dry form before mixing with water. If this is not done it would be considered "installer error".
2. Mixing the grout too thin (using too much water) will cause grout to mottle and dry in various shades. The thinner the grout the easier it is for the installer to install but the quality of the grout suffers. All grouts come with powder-to-water-mixing-ratio instructions. Unfortunately not all installers know that much about installing grout. If this is done it would be considered "installer error".
3. Any Portland grout can be ruined if too much water is used on the surface of the grout during the cleaning process. The color pigments can be washed away. Grout is to be cleaned with a clean damp sponge and never flooded with water during clean up. If this (flooding) is done it would be considered "installer error".
4. If the thinset used to install the tiles is allowed to purge upward between the tiles in some areas this can cause the grout to dry in various shades. The thinset will absorb the moisture quickly where it is present and cause the grout to cure too rapidly. But other areas (where thinset is not so present) the grout will cure normally and slower and a color change is possible. If this is done it would be considered "installer error".
5. Grouts can go bad. They can destabilize and "go off" in the bag without any hardening or indication there is a problem. Grouts have a shelf life. If old out of date grout was used this is considered installer error. The freshness of all grouts is easily obtainable and determining the suitability for use is the responsibility of the installer.
This isn't to say that a fresh product couldn't be bad but the truth is...it is very rare.
6. If grouts are not installed at the same time during the same atmospheric conditions color variations can occur. The manner in which the grout is installed is the responsibility of the installer.
As far as expecting perfection I would say if you aren't being totally anal and in fact have a problem then you should be entitled to better than what you have received at this point. However, you are dealing with an imperfect product by its very nature being installed by imperfect craftsmen who are only human. I wouldn't expect perfection but I am not excusing what has taken place based on YOUR report.
Lastly...the remnants of the old grout along the tile edges is inexcusable and just plain lazy on the part of the guys that removed the grout. All signs of a previous grout can be removed, and without damaging any tile.
I would be surprised if the store owner is able to shove the grout costs down the throat of the manufacturer in this case. Unless they know of problems with that particular run of product or unless they highly value the store owners business.
OK, that's enough for now. This should give you some knowledge of what you could be dealing with. Good luck.