I know that the "when played" abilities are optional, but what about the "resolution" and "end of combat" abilities? Do you have to resolve these even if you don't want to?

The reason I'm asking is because of the Daqan Lords Siege Machine card. It states "RESOLUTION: Discard 1 card from your hand to add +2 to this unit". Depending on the situation, I may not need to nor want to discard a card. However, all other cards I've reviewed would seem that you must perform the resolution ability and it is not optional. Also, the example in the rulebook says the player "may" choose to use this ability. Why would one card be optional but all others aren't?

Page 14 Under Abilities:

"Abilities on unit and tactic cards are always optional ... There are four different abilities: When Played, Resolution, End of Combat, and Action"

All four ability types are optional including resolutions abilities.