The idea is that if you pay them $1M, they won’t do their side channel earnings (and therefore reduce their potential conflicts of interest)
Both seems highly unlikely to me.
If you pay them $1M salary, then a 50k donation/bribe/dinner is less convincing.
This isn’t going to stop it, but perfect shouldn’t be the enemy of better.
Another is assuming that the price to buy support is fixed and the coffers are limited. The price might be $50k because perhaps that is the sweet spot on your target, but support and donations will go higher if needed and the special interest is motivated to garner support.
There are less "causes" that have the ability to fund a $300k bribe so even with that argument, we're still better off. (And it costs those causes more, bigger paper trails etc.)
They just hire someone to do their trading for them and buy an island to go along with their yacht.