Because that doesn't matter to the job. Having a degree is considered important.The subject of the degree is considered far less important or completely irrelevant. That might not be the best approach, but it is how it is.
That's far from the worst example. Roman citizens had the right of free movement within the empire and there were some good reasons for Romans from elsewhere in the empire to move to Britain. By the time most of Britain became part of the Roman empire, there were a lot of Romans with darker skin (all of North Africa had been Roman for a couple of centuries by that time). Travelling from north Africa to Britain wasn't a particularly big deal at that time - boat along the coast all the way, only the last couple of dozen miles with any significant degree of risk. I'm sure that some Romans moved to Britain and that some of those Romans had darker skin.
While it looks like the cartoonist who made the picture you quote was trying to portray a Roman soldier (and doing a pretty bad job of it, e.g. bronze muscle cuirass on a Roman legionary!) and there were a lot of Roman soliders stationed in Britain and some of them would have had families there and some of them would have retired there (hard to tell intended age from a cartoon, but he might be old enough to have served for long enough to retire) and some of them might have had darker skin (although AFAIK the legions stationed in Britain were all European), I think there would have been some Roman civilians who moved to Britain. There were some good reasons for doing so. Earlier on, there would have been good opportunities in the new province of Britannia as it was mostly Romanising. Lots of building work, for example, both civilian and governmental. Later on, there were times when it might be prudent for a Roman to get as far away from Rome as possible while remaining in the empire and Britain fitted that bill. The reign of Commodus comes to mind.
If they were depicting a very small minority of Romans in Britain having darker skin, it would be historically plausible.
Because the press said it was a minor incident and was just rumours going around facebook. So did the police. We'll also ignore the hundreds of people gathered either that night or the night after with police forming a barrier to keep the "English" lot away from the Slovakian areas trying to get retribution.