Maybe it would help to think of the credit as benefiting the solar industry more than wealthy people. It's a long road for new tech to reach economies of scale. If society has an interest in reaching that inflection point than the subsidy could be money well spent. Poor people will benefit when the price of the roof falls further; it may never have reached that point without early subsidy.

Perhaps it would be more equitable if Tesla received the subsidy directly. But that places the government in the business of choosing winners without the market signals of who would actually succeed if given the subsidy. Politically that's such a minefield. And no one ever lost votes for promoting a tax cut.