Should Charitable giving truly need an incentive. I really don't think it should. Either a person give or they don't it is up to them.
I sort of understand you situations to be honest. Though maybe I am not 100% sure. If by not allowing a person to deduct the charitable giving how would it hurt the government. Does not the government make the rules on taxes to begin with? So would it not be prudent to take it away and change it so it is most beneficial to them?
The point of my example was that if it is a role of government or society to maximize utility of society, namely in this case the less fortunate, then sometimes incentivizing donations might be a good thing. To hold this view that people should give charity because they want to or otherwise they shouldn't and that in turn they shouldn't be incentivized to donate might lead to less aid or the same aid at greater societal cost.
So then the question arises, is it more important to you to enforce your ethical position that donations should be given completely altruistically or is it more important that the unfortunate get more help and in a way that might cost society less? For me that question is simple- I want the latter as it raises utility more. I don't care if people are donating only because they are getting incentivized to do so if in the end it creates the best aggregate outcome. Besides, even "real", non-government incentivized giving may not in fact be altruistic.