Because in America, that statement is so heretical and antithetical to what the average American thinks America is, that most people probably wouldn't even be mad at you for saying it: They'd think you were legitimately unwell/insane.
It's on the same level as saying "I hate liberals and am not conservative".
Go work for a small business, have a fucking horrible experience, quit, and now you have the experience to back up the sentiment
Nah but for real I think you need to preface every discussion on small businesses with, “well of course I prefer small businesses to large multinationals, I’m just saying...”
And also you have to really be out as an anti-capitalist, you can’t say fuck small businesses but beat around the bush when it comes to capitalism. Because framing the whole thing as anti-small business while still operating in a capitalist framework won’t convince anyone, you need to state outright that the problem is capitalism which small businesses partake in.
Also you really need to come off as pro-worker more than you do anti-small business. Because as you say, the small business owner is sacred in US culture, criticizing them won’t get you very far. But framing your argument as pro-worker might chip away at the small business owner’s pedestal in the other person’s mind