From what I found on the Net, the connection between Sinterklaas and Spain has to do with our history. Even though Spain and The Netherlands were at war, they were still trading (among others oranges and herbs/spices).
It could also be that, when Sint Nicolaas was again buried in Bari (because the people were afraid that the Turkish muslims wouldn't respect the remains that were buried in Turkey), it came at some time under Spanish ruling ... thus making him 'Spanish'!
Further information: Sinterklaas and Saint Nicholas
In the Netherlands Saint Nicolas (often called "De Goede Sint" — "The Friendly Saint") is now aided by helpers commonly known as Zwarte Piet ("Black Peter").
The folklore of Saint Nicolas has many parallels with the Germanic mythology. Saint Nicolas has many resemblances with Odin, like the beard, hat and spear (nowadays a staff) and the cloth bag held by the servants to capture naughty children. Both Saint Nicolas and Odin ride white horses that can fly through the air. The white eight-legged steed of Odin is named Sleipnir. The letters made of candy given by the Zwarte Pieten to the children reminisce of the fact that Odin ‘invented’ the rune letters. The poems made during the celebration and the songs the children sing has to do with the fact that Odin was the god of the arts of poetry.
On the origins of the helper there are various explanations. The oldest explanation is that the helpers symbolize the two ravens Hugin and Munin who informed Odin on what was going on. In later stories the helper depicts the defeated devil. The devil is defeated by either Odin or Odin's helper Nörwi, the black father of the night. Nörwi is usually depicted with the same staff of birch (Dutch: "roe") as Zwarte Piet.
Another, more modern, story is that Saint Nicolas lilberated an Ethiopian boy slave called 'Piter' (from Saint Peter) from a Myra market, and the boy was so gracious he decided to stay with Saint Nicolas as a helper. In Belgium Zwarte Piet is still called "Pieter baas", derived from 'Piter'. With the influx of immigrants to the Netherlands starting in the late 1950's, this story is felt by some to be racial. Nowadays Zwarte Piet has become a modern servant who have black faces because they climb through chimneys and get blackened by the soot from the fire. The tools they held (cloth bag and staff of birch) are chimney cleaning tools.
Until the second world war Saint Nicolas was only helped by one servant. When the Canadians liberated the Netherlands in 1945, they reinstated the celebrations of Sinterklaas for the children. Unaware of the traditions, the Canadians thought that if one Zwarte Piet was fun, several Zwarte Pieten is even more fun. Ever since Saint Nicolas is helped by a group of Zwarte Pieten.
Presents given during this feast are often accompanied by poems, sometimes fairly basic, sometimes quite elaborate pieces of art that mock events in the past year relating to the recipient (who is thus at the receiving end in more than one sense). The gifts themselves may be just an excuse for the wrapping, which can also be quite elaborate. The more serious gifts may be reserved for the next morning. Since the giving of presents is Sinterklaas's job, presents are traditionally not given at Christmas in the Netherlands, but commercialism is starting to tap into this market.
The Zwarte Pieten are roughly to the Dutch Saint Nicolas what the elves and reindeer are to America's Santa Claus. According to tradition, the saint has a Piet for every function: there are navigation Pieten to navigate the steamboat from Spain to Holland, or acrobatic Pieten for climbing up the roofs to stuff presents through the chimney, or to climb through themselves. Throughout the years many stories have been added, mostly made up by parents to keep children's belief in Saint Nicolas intact and to discourage misbehaviour. In most cases the Pieten are quite lousy at their job, such as the navigation Piet (Dutch "wegwijs piet") pointing in the wrong direction. This is often used to provide some simple comedy in the annual parade of Saint Nicolas coming to the Netherlands, and can also be used to laud the progress of children at school by having the Piet give the wrong answer to, for example, a simple mathematical question like 2+2, so that the child in question is (or can be) persuaded to give the right answer.
Santa Claus, as known in the US (white beard, red and white outfit, etc.), is an entirely other person, called (de) Kerstman (trans. (the) Christmasman. Although Sinterklaas is the predominant celebration in the Netherlands in December (36% of the population only celebrates Sinterklaas), Christmas is celebrated by a fifth of the Dutch population (21% celebrates only Christmas). 26% Of the Dutch population celebrates both.
Here's another link about the Celebration: http://en.wikipedia.org/wiki/Sinterklaas
Patron Saint of (among others) children, prisoners, seamen and Russia