Not always. NightOwl's ToS said "We reserve our right to alter the terms in this Agreement and/or the pricing information and method detailed in NightOwl app's website at any time. In case the Agreement is amended as described, we will post an updated version of it in our website, at which time it becomes active and binding."
That's probably illegal, but plenty of sites and apps will try it anyway.
That's probably illegal, but plenty of sites and apps will try it anyway.