Why are teachers forced to choose?

The following article recently appeared in the Washington Post: http://www.washingtonpost.com/wp-dyn/content/article/2008/09/23/AR2008092303835.html. It talks about the new system proposed by the chancellor of schools in Washington DC to try to compensate teachers more. Unfortunately, it requires teachers to make a difficult decision - either earn a higher salary but forgo tenure, going on a one-year probation or earn a lower one and maintain tenure. I have my own doubts about the merits of the tenure system but why do teachers have to choose in order to be highly compensated for the difficult work that they do. I would be interested to know what the process is for letting a teacher go after their one year probation if they forgo the tenure. Although I know in corporate America, there is no such thing as tenure, I also know that any reputable company has a system for firing an employee which, beyond actually committing a crime, for the most part leaves the burden of proof to the employer to make a case for firing the employee (except in the case of layoffs which there are plenty of these days :)). I just feel that there must be some way to provide fair, reasonable compensation to teachers without putting anyone in these uncomfortable and tension-laden situations.