Most chareidim already do ‘national service’ in one form or another – we just call it chessed. Local neighborhood directories are filled with gemachim and chessed organizations, many of which are quite time consuming, and the government doesn’t pay a cent for it.
The concern is not about helping out. The concern is the secular government telling us when, what, how, and with whom.
At the founding of the state there was a deal made between b-gurion sr”y and the Chazon Ish Zt”l. They are breaking the deal.