[citation][nom]rozz[/nom]well - its not really their fault dude.. its USA's terrible laws[/citation]
Actually, it has more to do with the laws of other countries than US law. US law allows for sweepstake type contests as long as the participant does not have to pay a fee in order to enter. If you had to pay a fee to enter, US law defines that as gambling and is prohibited. This is why in the US you can't be forced to make a purchase in order to enter.
Other countries define gambling differently and thus have different regulations regarding contests. For instance, Canada and many European countries require participants in contests to solve some sort of puzzle to enter, making it a game of skill in order to avoid breaking their gambling laws. Since Tom's only requires you to apply for the contest, they would be illegal in many other countries. With so many countries in the world, this becomes a legal nightmare. This is only one of many reasons that worldwide contests become difficult and expensive.
Another reason is the various countries tax laws. In the US, when you win a prize, contest sponsors need to send the proper tax form to both you and the IRS and you will need to claim the value of the prize as income and pay the appropriate tax. It becomes very difficult to deal with all of the different tax laws of other countries.