Honestly, when I buy a video or a DVD or a Blu-ray, I own the physical unit. Just like I own my table or my shoes, I have paid with my money to own this video or disc. I can't own the ideas on the unit, as they are copyright protected (that is of course a whole other discussion), but I own the physical unit the ideas were transferred to. I can do whatever I want with that video or disc, so long as I don't use it to break copyright laws.
That's the thing, though . . . in order to not break copyright laws, you have to look closely at the rights the copyright holder has transferred to you.
All that crap at the beginning of a disc, videotape, etc. that says something along the lines of "Licensed for home use, no public performance" (I'm not direct quoting because I never pay attention either) spells out the extent of that license.
So, if you buy a copy of Prince Caspian
at Wal-Mart and then show it at school (if you're a teacher) or church or what have you, you're technically breaking copyright law, because Disney didn't give you performance rights. That's the entire reason this website exists:
You have to buy a version that contains the rights to do what you want to do.
In the case of ripping DVDs for digital copies, I'm pretty sure that the same warnings at the front of a DVD we don't pay attention to, also say "No Duplication or Reproduction."
As for why Disney isn't participating in Wal-Mart's new venture, I assume it's because they don't want to cannibalize their market for this service: