I'm not a well driller so cannot answer that but will take a stab at it. I don't think they intended it to be a mud well. They originally welded on a drive shoe probably with the intent on setting it into the bedrock in anticipation of it being a rock well. I think they drove down the casing as they went, not knowing the formation until after they passed it. I'm guessing they didn't want to pull up all the casing to add screen to the last 5 feet of it and thought the glacial till would be stable enough to be the screen.
They did not actually develop the well and only pumped it at 5 GPM to satisfy the minimum requirement. After I built the house I put in my 10 GPM pump, it sucked in the mud and I had to get the driller back to flush it out and set the gravel pack. It filled back in with mud again and I augered it out by hand and developed the well myself after the driller did a bunch of no-shows.
That sounds reasonable to say, that they didn't plan on making that type of well from the beginning....they are probably normally rock well drillers, and didn't know how to react to that sand/gravel formation. It is normal to install and grout a casing and then install a well screen. Sounds like you've made it work. If you ever have problems, you can easliy install a reduced diameter screen in the 6" well with a packer assembly. If you were really concerned about sand/gravel intrusion, you could even install a prepacked screen (a screen in a screen with a proper gravel pack).