It's non-authoritative, clients can slap a
"cannot verify this value, check the price on the next step" sticker on it as
necessary.
Because users are well known for carefully reading and following instructions, rather than
just going ahead and hitting the OK button without hesitation.
This isn't a solution, it's just a way to remove responsibility and then blame the
user - "it's your own fault for not reading, you were warned" - which may be
correct, but it's entirely predictable and shouldn't be encouraged through bad
design.