ISPs are at best a oligopoly in some instances a monopoly. That was not true earlier in the decade. Government always regulates when that happens. Also you have ISPs now owning content which gives them incentives to slow down certain traffic. That is why we need net neutrality.
Why should a company not be allowed to give preferential treatment to their own products?
Because the ISP is a monopoly or oligopoly. There is no competition to keep them honest. They could slow it down so much that it becomes unreachable.