Do you mean that "entertainment" is an uncountable noun?
Besides, I still don't see why this sentence doesn't sound good--To a lot of teenagers, seeing a movie is a good amusement.
I suppose it sounds awkward because in Canada we don't really consider the movies as an amusement. So we would never use amusement in this way.
I don't really know why - To a lot of teenagers, seeing a movie is good entertainment. - does not have an article before good entertainment, other than to say it doesn't sound right to me. Sorry to give you such a poor explanation.