I guess it is because Timers can be created only via TimerService. The Stateful beans are allowed to access a Timer ( for instance obtaining it as return parameter from a stateless bean's business method). If you compare the table 4.4.1 (operation allowed in stateful beans) with the 4.5.2 (operations allowed in stateless beans) you will realize, the stateless bean methods can access TimerService in addition to Timer.
Hope this helps.
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