First, you can use oil that is a 'wider' range than mfg recommends, ie 5w-40 instead of 10W-30. Just do not use oil that has a 'narrower' range, 20W-30 instead of 10W-40. The first number is the oil viscosity when cold and the second number when hot. If you ever want to really see the difference get a quart of straight 30W oil and chill it and your 5W-40. Pour them see the difference.
The biggest contributor to engine wear is cold starts. I want to have as low cold viscosity as I can since I frequently start the bike in below freezing weather. That's also why you see mfgs recommend a different oil for really cold weather.
Second, as to soaking the customers. Yes and no. The diesel varieties are made for bulk distribution so are cheaper. The oils were designed for diesels. Shell found that T6 also met the JASO MA standard so they stamped that on there. Mobil did the same, but, I have not found their diesel blend in anything smaller than 55gal drums. Mobil developed two blends specifically for motorcycles. Much smaller quantity production so much more expensive.
Could Mobil just repackage the diesel product and sell it cheaper than the motorcycle oils? Maybe, if their production line is easily reset for it. It is all a numbers game. I would bet that Shell really does not see much profit in the T6 sales to regular consumers. I also suspect that they packaged the T6 in smaller quantities for people who own diesel pickups and do their own oil changes. Mobil probably does not see that big a sales number to justify retooling a production line for smaller quantities.
Do you think Mobil just takes the diesel product, pours it in quarts, puts a motorcycle label on it and charges twice as much? I don't think so. I suspect there is a different production line for the motorcycle oils with a slightly different blend for dry and wet clutch models. Nope, they are not the same as the diesel oil (I have looked), even though the diesel oil meets the JASO MA standard.
If you want to check go look at the complete listing of specs for each of those oils and see what differences there are in the contents.
Like I said before, oil and filters is one area I don't buy based on cost.
PS will it hurt your bike to run T6 or any other JASO MA certified oil in it? No. That's why that standard exists, just like all the standards for cars and trucks. Use an oil that meets the mfg standard and you'll be fine.