Assuming you could nail down who is truly qualified for this plan, giving away service is probably not a good idea as it drives up usage of cell towers. This could lead to slower service for customers who pay the full amount.
Could the carriers implement service of lower quality for people who pay less - and make this discounted service available to the unemployed? Perhaps. If you can make it easier for this group to get phones, perhaps there will be incremental revenue from text messages, ringtones, etc.
For fascinating analysis of why wireless carriers should/should not give away service, check out this article.