You're missing that for this to work, miners, those who invest into processing bitcoin transactions, would be willingly lowering the value of what they do. Other comments bring issues they'd already face in cost trying to adopt banning more transactions. But if they accepted those costs, bitcoin's reputation of stability and trust would be broken, if 1 BTC = 0.75BTC because .25 was from tainted transactions, nobody wants to risk that happening to them so they'd abandon quickly and the price would plummet. But if the price would plummet then miners would be inclined to do the opposite to avoid that happening.