WoWInterface - Button:SetScript

I've been scratcing my head for a while trying to figure this out.
This code works:

Lua Code:

local btn = CreateFrame("Button", "DoTrackerLockBtn", menu, "UIPanelButtonTemplate")
btnLock:SetText("Lock")
btnLock:SetWidth(80)
btnLock:ClearAllPoints()
btnLock:SetPoint("TOPRIGHT", -110, -16)
btnLock:SetScript("OnClick", function() print("test") end)

This code works aswell:

Lua Code:

local function printTest()
    print("test")
end
 
local btn = CreateFrame("Button", "DoTrackerLockBtn", menu, "UIPanelButtonTemplate")
btnLock:SetText("Lock")
btnLock:SetWidth(80)
btnLock:ClearAllPoints()
btnLock:SetPoint("TOPRIGHT", -110, -16)
btnLock:SetScript("OnClick", function() printTest() end)

But this code does not work:

Lua Code:

local function printTest()
    print("test")
end
 
local btn = CreateFrame("Button", "DoTrackerLockBtn", menu, "UIPanelButtonTemplate")
btnLock:SetText("Lock")
btnLock:SetWidth(80)
btnLock:ClearAllPoints()
btnLock:SetPoint("TOPRIGHT", -110, -16)
btnLock:SetScript("OnClick", printTest())

Why is this? It seems to me that in all three cases I send a function to the SetScript Method but the last on does not work. What am I missing?

Thanks in advance
Marcus Larsson