...I don't know that much about poisons, but I bet the people in this lob have whole arsenal of "stealth" poisons that could much more easily go unnoticed. And if you want to make it obvious, why not go for something even more obvious or simply a bullet?
It seems more like an "artist's signature" thing and it would be interesting to know who this "artist" is!
It's a very rare agent that only very few assassins have access to, so most routine poison searches would not pick it up. Especially since the symptoms are basically nondescript. In addition to that polonium was pretty much unknown as a poison when Arafat was killed - the high profile Litvinenko killing was after Arafats death. The lethal dose is extremely low and even if it is detected, there's no antidote.
That's a pretty good stealth poison in my book.
It's a very interesting article about what happened to Litvinenko and how polonium poisoning actually works.
You're right that it's a curious choice in some respects. In the Litvinenko case, police were literally able to retrace the steps of the assassins by following the radiation. Its short half-life also suggests that whoever was behind it had access to a nuclear reactor.
That same reason also makes me somewhat sceptic in this case, but who knows. Hopefully, time will tell.
With Arafat, the assassin almost got away with it unnoticed. There could be other cases of Polonium use as well which were never reported, it's remarkably tough poison to detect.