No suggestion here that the seat was inop at the time OP selected it. Could have gone inop as late as the inbound to LGA. In theory, someone should have let the GA know so that the GA could have found OP and found another seat immediately, but that level of communication doesn't occur on any short-hauls of which I'm aware.
UA would refund OP the E+ fee if OP had paid one. But, it was a freebie as a Silver, so there's no refund. If OP wants to complain, I would make it 1/3 the length of the post here and OP may see a few miles.