There must be loads of jobs someone with an minor injury could do in a supermarket, stock sheets, staff rota's, supervising self service tills, doing the announcements.
None of those are in any way appropriate "alternative" jobs for an injured retail colleague.
Stock sheets are done by the warehouse and/or the relevant shop floor department manager. A checkout colleague would not have the training necessary to have any control of that, and it is a small part of someone else's job that would not occupy them for an entire 8/10 hr shift.
Staff rotas are done either by the department supervisor or department manager and there's no way they would give the job to a checkout colleague - both because they don't know the system and because the algorithm used to allocate specific shifts will involve a lot of different factors, including confidential information known to their direct supervisor but that other colleagues have no need to know (eg knowing that Pat can't do a self scan shift because she has a dodgy knee and can't stand on it for that long or John never does Monday mornings because he has counselling sessions at 10am) Also, again is a minor job that would not take up an entire shift.
Self Service attendants are still required to use their hands - collecting baskets, carrying items customers have changed their mind about back to the collection point, getting bags for customers, helping customers with awkward/heavy items, having the dexterity for the screens for when you have to put your colleague number in to authorise an override, be able to change the till rolls which is trickier in a self scan than a normal till and they all seem to run out at the same time etc
Announcements ... in my store that is the job of the Customer Service Desk and again is a very small part of a much larger job - I have just finished a 10 hour shift on the desk in my store. I spent maybe 10/15 minutes over the course of my entire shift making announcements and that time is only that high because I was on the late shift so had to do the closing announcements and also the calls to inform customers of the late night mark downs. In an average day shift you'd rarely spend more than 5 minutes making announcements, they really don't need someone sitting there just in case they need to call someone to the checkouts or someone lost their child.