this is how the idiom works in this case, simple as that.
think of
to=with
in this context
it's like inviting yourself
to a nice place (restaurant, etc) and enjoying it
to treat
to provide (as another person) gratuitously with food, drink,
entertainment, or some other source of enjoyment or gratification
especially as a compliment, gesture of kindness, or bribe --
usually
used with to <he
treated her to a strawberry soda> (2)
: to provide (oneself) with a similar source of enjoyment or gratification --
usually used with to <
treated herself to a new mink coat>
c :
to provide with something that is or is held (as in irony or for
amusement) to be a source of pleasure or gratification <the
Americans were
treated to a remarkable display as the Tripolitan ship blew up -- C.S.Forester> <when he punished he
treated the culprit to ten minutes of biting irony first -- Storm Jameson>
M-W unabridged