When using GridClientSelectColumn, AllowRowSelect and AllowMultiRowSelection multiple selections can be made only by click the checkboxes not the rows.
Steps to Duplicate
1. Goto the following URL
http://demos.telerik.com/aspnet-ajax/grid/examples/columns-rows/columns/column-types/defaultcs.aspx
2.Click all 3 checkboxes one after the other. Notice that after doing so all 3 checkboxes are selected.
3. Uncheck the checkboxes or refresh the page.
4. Now try to select all rows by clicking on the row, not the checkbox. Notice that only one checkbox can be selected.