This argument doesn't hold. The effect that you are shooting for is that you minimize the damage to the planet. Having some people use the grid as an 'unlimited free battery' is a fantastic way to achieve the exact same effect as if all those people invest in large physical batteries without the investment and without the environmental impact.

This sort of thing should be incentivized rather than discouraged because of the large scale effects, not because of petty accounting rules or by creating jealousy where there isn't ground for any: poor people without panels aren't subsidizing the rich, they are effectively not subsidizing the energy companies, which are far, far richer than the people with rooftop solar. It's citizens generating power for other citizens at the price that the market dictates. The thing to hope for is that enough of this will eventually lead to a reduction in retail prices rather than that we allow the energy companies to make money off surplus from one citizen sold to the next.

Batteries are nice, reduced grid generation requirement during daylight hours is even better. And that's before we get into fire risks of large battery installations, space requirements, depreciation, recycling and so on.