By fasting against deals made by the Sri Lankan government with the Tamil Tigers, such bhikkhus are not practising the Middle Way as presented by the Tathagata, which guided all Buddhists to avoid the extremes of self-indulgence & self-mortification.
A broader point regarding the politicization of the sangha in Sri Lanka is that it distracts such monks from the main purpose of being a monk - walking the Eightfold Path towards Nirvana, & sharing one's insights with other Buddhists, so to help them in their search for enlightenment.
In the Tipitaka, the Buddha discouraged bhikkhus from having any participation in politics, directing them to focus on dukkha, & the ending of dukkha, rather than worldly matters. If such monks are indulging in their strong-held political views & biases, what are ordinary lay Buddhists going to end up doing in this strife-ridden country?