I'm not sure which phenomenon you're asking about. I see two: a) that the worksheet cannot be sorted while it's protected, even though you specified that sorting should be allowed, or b) that Sharepoint allows you to edit it without making you check it out. Or maybe there's a third that I'm missing.

If it's the second one, I'm not a Sharepoint expert either and anyway this is an Excel forum so it isn't the place you would necessarily expect The Right Answer. But I've worked with Sharepoint ... sometimes ... and I vaguely remember that I could edit a sheet without checking it out, but in such a case I risked conflicting with a teammate. I'm not sure, though.

If it's the matter of sorting while protected, my mind immediately focuses on the fact that you say some cells are merged. I've definitely encountered this; if I merge cells between rows then Excel will not allow me to sort row-wise, and I expect that if I merge cells between columns then it wouldn't allow me to sort column-wise either. I haven't found a way around that.

But if you've merged cells between columns—say D22 and E22 are merged, but not D23 and E23—then I don't think that should prevent you from sorting. If this doesn't already tell you what's going on, then rather than guess I think I need to see the real thing; are you willing to post the workbook? If there's confidential information in there, you may have to sanitize it or go to the work of creating a facsimile instead.