I think the situation people mostly worry about with market making is where Alice submits an offer to sell for at least $1.00 and Bob submits an offer to buy for no more than $1.02. There's $0.02 of surplus here, and we would expect it to get somehow split between Alice and Bob. When we add in Martin (market maker) to the scene, people worry that he would accept both Alice's and Bob's offers and pocket some of that $0.02 surplus for himself. If Alice and Bob both put in their orders at the same time, then Martin is obviously left out, but it's never really simultaneous. This leaves a time window when Martin could potentially insert his own pair of buy and sell orders. If it's a rather slow market, and Bob's orders appears a few days after Alice's, it's reasonably clear what service Martin is providing (Alice can quickly and reliably offload this thing she no longer wants, and the item is readily available for Bob to buy when he finally wants it).
On the other hand, if the market isn't so slow, and Martin is doing HFT, it's less clear what Alice and Bob gain from him. If he's able to take a position and close it in a matter of seconds, people start to wonder just how much better off Alice is from having been able to finish her sale seconds earlier. It looks like we've got diminishing returns as Martin's holding periods shorten. His claim that he's providing liquidity looks dubious in a market that has to already be pretty liquid to support the activity people see from him, so they suspect they're being hoodwinked. Modern technology enables really large scale market making, so Martin's got a lot of money flowing in, which makes people even more suspicious.
Personally, as a rather small-time investor, I don't really fret about this. Even if it is the case that I am getting some of what would otherwise be my money skimmed off by someone claiming to have provided liquidity that was already present, I don't make enough trades for that potential loss to matter much. I worry more about whether this is a good (for society) use of the math/engineering talent we produce.