Well, first of all, I don't see a problem with this decision, it's pretty common sense. Why dispense two big internal stations for a short range air-to-air missile? You should use them for BVRAAMs or AS weapons.
It's better to carry ASRAAMs externally than the JDAMs or anything else. I doubt though that this option will be used a lot, as two self defense missiles will do in most instances. Two medium range missiles is still a bit short, esp. when you need to carry JDAMs as well... the bombs will need to be carried externally then, I'd guess? Anyway it's not so much a requirement for the role the RAF/RN needs these planes, and luckily the ASRAAM has quite some range.
Also, if you need to carry a TGP/LD pod and maybe additional jamming/countermeasures, you might as well put two ASRAAM under there as well.
In the end, external tanks will need to be slapped on as well, tankers are in high demand and short supply, conformal tanks are most likely impossible on the JSF, especially on the F-35B. Wake up, the wings will be used anyway guys! Ok, not on the stealthy, covert airspace penetration missions in which stealth is vital and on-station time minimal, but if you need more than two ASRAAMs on those missions, you're already screwed by the intel guys.
Argh, this plane could have been way better than it's going to turn out... but well you know what that means for the F-22 and arguably cost/time. I'm confident it will still be the best though for one-tier fighter fleets, or mixed with the F-22.
The external ASRAAM fit will be common across all three JSF variants and could therefore attract interest from other international customers, who will otherwise be tied to Raytheon's AIM-9X Sidewinder.
The F-35A still has six internal stations right? (pls someone confirm, as I'm worried it might be altered to save costs) With six internal missiles, it wouldn't need to carry external ASRAAMS. As for other versions, dream on, they'll settle for the AIM-9X, only likely exception is Israel because it has its own missiles. ASRAAM will make a JSF buy more expensive and most countries already have a hard-time selling this investment to it's people. They'll stick with the standard, cheaper AIM-9X. Ow, and Uncle Sam will 'convince' you it's better.