To be fair the MD schools have opened practically as many new schools as osteopathic schools in the last decade. For us it means more because we only had 20-some schools a decade ago and they had 120+, but in terms of raw number of new schools, they're practically the same. Our class sizes are bigger and tuition greater, however, so ultimately new DO schools are making more with less effort as you said.
I have no problem with the idea of paying to cover the expense of students for rotations, but my point is more that the measure of fairness and good rotations isn't really based on how much a med school pays to send their students on rotations. Like I said, plenty of MD schools don't inherently pay for rotations. It's one of the reasons MD schools in NYC were up in arms about the contract St. George's had with the NYC hospitals, paying them over a million for their students to rotate, in turn displacing US MD (and DO for that matter) students. That doesn't mean those MD school's rotations are inherently worse.
Our rotations are worse, not simply because we don't pay sites, but because our schools don't put money and effort in to creating a standardized and effective curriculum in 3rd and 4th year. We are basically sent off to preceptors or hospitals, some of which have barely any experience teaching, with very little guidance as to our objectives, requirements, etc. On top of that our clinical departments consist of less than a handful of administrators covering hundreds of us. Sure we're going to smaller sites, but I've rotated with a bunch of MD students even at some of these smaller sites, and the main difference is that they actually have school-provided study resources and an outlined curriculum that we generally lack.
Also, I will say that direct payment for students isn't the only way schools can "reimburse" hospitals for their rotations. My school doesn't pay sites for rotations, but they put money and resources towards establishing affiliated residencies at those sites, which end up bringing in ~$150k/yr/resident. Now most of that is used up in the residency, but the non-allocated institutional money and a bit of that allocated money makes the effort valuable or in many cases lucrative for a program that didn't previously have residencies. They also save on hospitalists once the residents are established and further along.