I would just do:
DECLARE @TempTable TABLE(ID int IDENTITY, Charges varchar(270));
INSERT INTO @TempTable
SELECT ID, Charges FROM TicketType WHERE Charges <> '' and @inItemType=1
UNION ALL
SELECT ID, Charges FROM SeatType WHERE Charges <> '' and @inItemType=10;
A CASE
expression, in common with most expressions in most mainstream languages, computes a single (scalar) value. It cannot return tuples or (as in this case) entire rowsets.