I think it might be seen as a "cheat" because everything seems to resolve so
neatly:  Jim is freed, not only from his captors, but from slavery.  Huck is
"freed" by Jim's revelation that pap is dead.  And Huck's struggles with
social mores are "solved" by his resolution to "light out for the
territory."  So all seems right with the world and all the issues raised by
the book seem resolved by a happy ending for all.

I'd argue that the ending is not really as resolved as it seems, but it does
seem to tie everything up in a neat little package, a resolution that feels
like cheating at first.

Sharon