Why do L8+ agents attack portals but then do not claim it

I have noticed that some agents (mostly L8) will attack my portals but then they leave them unclaimed. Is this some kind of strategy or do they run out of XM? This has happened many times so the latter is unlikely.

The reasons could differ from agent to agent. Sometimes it could be because the agent is on a rush, maybe dropping xmp while driving or while on the bus, other times because you don't have resonators of the appropriate level (L4 resonators for example) and you prefer not to waste L8 and L7 resonators on an incomplete portal that will be easily destroyed 5 minutes later.

Often high level agents do this to help low level recruits to gain AP, but everytime I've seen doing this they put at least a L8 resonator to make an eventual attacker waste at least a couple of XMP to regain the portal.

As far as I know there's no actual strategy behind this, just personal decisions of the single agents.

Kappei's answer may be correct, but in terms of roleplaying this is more suitable:

The Third Faction

There are rumours that there is a third faction, its colour is white. Anyone can be part of it, be it a green or a blue player, since it's not about the colour but a state of mind.

• There isn't actually such faction, but players who tend to leave neutral portals behind often claim they are part of this non-existent faction.

• These players are the most honourable players ever, their lives and gameplay resembles lives of Hermits.

• Affiliation with this faction isn't for everyone and is highly prestigious. (It's not something to be ashamed of!)

• When a player acquires certain knowledge, or one could say 'enlightenment' (do not confuse with the green faction), he may decide to join this faction and fight for their beliefs by purifying all portals.

• A player joins this faction usually when he has reached his mission and no more desires to be an agent, such as accumulate 20M AP, submit 500 portals, walk 2000 km, visit 10k portals etc.