I'm not certain that there was any legal compulsion to build stations near turnpikes. Simmons and Biddle's "Oxford Companion to British Railway History" mentions that turnpike trusts were considerably disturbed by the coming of the railways. The Trusts usually objected to a line being built in their vicinity to the extent that by 1834 a standard printed form was provided by which a Trust could lodge their objection to Parliament regarding any proposed railway!
Bearing in mind the considerable reluctance of successive UK governments to control the spread of railways, I think it far more likely that the compulsion to build a station near a turnpike came from the commercial desire of the railway company to gather in passengers from the much slower and more uncomfortable stage coaches. It may be possible that Acts for particular railways included a reference to a local turnpike trust's road for perhaps local distribution or collection of goods?