In order for NASA to actually do the plan, not something else, it needs to be directed. Unless you claim mind-fiat of Obama or the NASA directors the plan probably will not happen. Therefore, it has to go through Congress in order to guarantee the plan happens, which is where normal fiat comes in.
As for normal means, if you do not expressly say ~"funding allocation" in the plan text it would force NASA to redirect its funding from another project into your plan. That risks linking into a tradeoff disadvantage.
In summation, funding and tradeoff are both opportunities to incite anger in Congress and get a unpopular link, but very few plans have a direct link to the politics disadvantage.