If enrolled in Ms-PhD, do I have flexibility not to progress beyond MS?
(Copied from my comments)

Firstly:


In some cases the terminal MS program costs you more because you are expected to only do coursework, whereas in the MS-PhD program you are expected to service the department as a research or teaching assistant. 
I would expect that if a school offers both options they would frown upon your entering the PhD track with the intention of quitting after the MS 
Most importantly, this is something that you can and should ask the administrators at the schools you are looking at. There's a good chance that the policy is different school to school. 


To answer your second question, it depends on what you mean by consequences:

If you meant only financial ones: I've known many cases (not in CS) where students quit after the masters, and in none of those cases were they asked to pay the school back. Mostly because of point (1) above. In fact, some departments offering the MS-PhD program seem to take it as a given that some students will drop out. But depends on what you want to do, there could be non-financial consequences: People who've invested time in you may be reluctant to write you recommendation letters etc.