It has not just been those PSV fans in Madrid. There have been several incidents across the continent in the past few days where soccer hooligans went out of their way to mock and degrade beggars.
In Madrid, those PSV individuals were throwing small coins and openly laughing at the beggars in Plaza Mayor.
In Rome, a Sparta of Prague follower pissed on a beggar. And in Barcelona, a group of Arsenal fans openly mocked a beggar in the street (with some reports saying that they burned banknotes in front of him).
Be aware that not all those beggars were Romani, either. The only ones that apparently were (Romani) were the ones in Madrid. The others were locals.
I think it is not so much that the hooligans thought “these are gypsies and they have a possibly deserved horrible reputation”, but a more general “beggars are not worthy of pity” attitude. An article in a very respected Spanish newspaper posits that this kind of actions came from thinking that “poor people are poor because they don’t want to work and pull themselves out of their situation, and thus are not deserving of charity”. It may have been a factor, as well, that all those hooligans were from another place and would not expect to hang out at Madrid, Rome or Barcelona for long.
All in all it has been sad, really.