I tried your code in the rails console in a new rails app (3.2.15) with sequel added (4.6.0) and I did not see that behavior. Not sure what, but something is borked on your installation:
db = Sequel.sqlite
# => #<Sequel::SQLite::Database: {:adapter=>:sqlite}>
db.create_table(:t1){
String :f1
String :f2
}
# => nil
db.create_view(:v1, db[:t1].select(:f1))
# => nil
sel = db[:t1].select(:f1)
# => #<Sequel::SQLite::Dataset: "SELECT `f1` FROM `t1`">
p sel.sql
# "SELECT `f1` FROM `t1`"
# => "SELECT `f1` FROM `t1`"
p sel.columns#[:f1]
# [:f1]
# => [:f1]
sel = db[:v1].select(:f1)
# => #<Sequel::SQLite::Dataset: "SELECT `f1` FROM `v1`">
p sel.sql
# "SELECT `f1` FROM `v1`"
# => "SELECT `f1` FROM `v1`"
p sel.columns #[:"`f1`"]
# [:f1]
# => [:f1]