I agree with you, not only because couples do indeed trust each other, but also because it may come off as mistrusting to ask your s.o. to use something like this.
However, I think there might be a market for this with casual hookups. In these situations, you may want to let someone access private videos/pictures, but don't trust them enough to just send it, especially if your face is included. Receivers might also be more inclined to pay. Additionally, while I think it would be acceptable in this situation to ask someone to install an app, the makers of this have to take extra care to ensure that this looks serious and doesn't come off as a scam.
However, I think there might be a market for this with casual hookups. In these situations, you may want to let someone access private videos/pictures, but don't trust them enough to just send it, especially if your face is included. Receivers might also be more inclined to pay. Additionally, while I think it would be acceptable in this situation to ask someone to install an app, the makers of this have to take extra care to ensure that this looks serious and doesn't come off as a scam.