There are certain rules in place, but they are aimed at higher ranked players. In particular, the top 30 at the end of the year are 'Commitment Players'. The following is from
A 2012 ATP commitment player is any player positioned in the Top 30 in the South African Airways ATP 2011 Rankings (singles) as of November 14, 2011.
The commitment for the commitment players is, the singles event of all ATP World Tour Masters 1000 tournaments for which he is accepted, the Barclays ATP World Tour Finals (if qualified as a direct acceptance or designated as the alternate) and four (4) ATP World Tour 500 tournaments, one (1) of which must be held following the US Open. For commitment and ranking purposes, the Monte Carlo Masters 1000 will be included in the minimum requirements for the 500 category. 2012 Davis Cup points may be counted as one (1) of the best of six (6) in the ATP World Tour rank- ings, however, it shall not count towards the commitment requirement of a commit- ment player.
The 4 Grand Slams are run by the ITF, and I don't think any player are obligated to play, should they choose not to do so.
From a tournament perspective, the organisers are far more interested in the higher ranked players. They are the ones that will pull in a crowd, and make the tournament a success. In smaller tournaments, top players are even paid 'appearance fees' in order to lure them.
The reality is that lower-ranked players have little leverage when it comes to payments. The Grand Slams are where they can potentially receive their biggest pay day: the size of the draw means they have a better chance to qualify, and at the AO in 2012, first-round losers took home A$20k. Whilst a boycott at such an event may give them the highest exposure, it would also cost them.