I think that there's a couple of things to think about here which means that this is, whilst not run of the mill, certainly not a major deviation from the norm.
It was the Guard who intercepted it, not exactly the most rational and calm members of the Iranian military. Ordinarily the standard Iranian military is fairly calm about US aircraft in the region, they have to be, they exist in one of the busiest American flying zone outside of America itself. EP3s, Hawkeyes, drones, and probably aircraft that don't 'exist' have flown or are flying over or near Iranian airspace. If you flipped out over every US drone, you'd run out of missiles. Not only that, but you've got Iranian maritime aircraft and unmanned drones monitoring the US navy in the area, so it's quite a busy airspace. I certainly would not like to be their air traffic control.
Chances are, the IRG thought the drone had crossed into Iranian airspace, or perhaps they just didn't like the way the drone looked at them. These guys are Islamic hotheads, they don't like Americans or American equipment, so if they could think of a reason to shoot at something American they're not going to hesitate.
Secondly, they were flying Frogfoots. Which (Thank God and Allah) is their most formidable aircraft. The Frogfoot has two Aphid AAMs and the cannon, that is their entire Anti-Air armament. It's a Ground attack aircraft, so it's not really up to the role of shooting down aircraft but if they had really wanted to shoot it down, they could have used their Aphids, they can take down Predators, they have done so before:
Alternatively one could argue that they wanted to save their A-60s for a potential war against America and thought that the Predator would make a simple Gunzo target, in theory it would but then you have to take into account the speed difference in approach between a Predator drone and an Su-25...which, assuming this is a MQ-9 at cruise speed, is about 230mph.
I don't know what the stall speed of a Frogfoot is, I imagine it's not very high given its ground attack role, but it could be a struggle to stay in guns range in an old aircraft with little gun aiming technology above 1970s tech. Plus, I'd wager that the MQ-9 is no slouch in the maneuver department, I don't know what its turning circle is, but I'd put money on it being smaller than a Frogfoots.
If it was an MQ-9 it would also explain why it didn't shoot back, because they're still working on bolting some Stingers to it, alternatively it could have been a standard MQ-1 under Weapons Tight ROE. Better to have a drone shot down into the sea, than kill two Iranians and risk sparking off a major incident.
There's also the possibility that the IRGAF pilots aimed to miss, and were firing warning shots. This doesn't tally with their reputation as hotheads, but it's a sensible reaction to a perceived airspace violation.
On that note though, it also cannot be denied that there is a possibility that the drone WAS in Iranian airspace, but not deliberately, given that Russia has flogged Iran a fair bit of drone spoofing technology, the drone could have been fed false co-ordinates by the spoofer and strayed over the line, but one would have thought that if this was the case the Iranians would have brought forward flight data from the Frogfoots to prove it, the fact that they haven't does tend to indicate that the incident occurred in international waters as the US has stated.
Not the first time a spy plane has been shot at or hindered on a mission. Remember the Hainan island incident in 2001? Or the harrassment of the Impeccable in 2009, heck, even the Pueblo incident.
At least this time no-one was hurt or killed. That is the beauty of drones, I've got to admit that.