ASK A QUESTION what's the difference between con tal de and con tal de que?
are below examples typical to distinguish between the above 2 phrases: with con tal de + infinitive & con tal de que + subjuntivo.
- Me da lo mismo el sitio, con tal de poder descansar...
- Me da igual el alojamiento con tal de que sea limpia.
I'm not quite sure what you are looking for as you seem to know that the infinitive is used after a preposition and that the conjugated subjunctive verb occurs in a subordinate clause beginning with que.
Are you asking about the necessity to have a subject change to use the subordinate clause rather than the infinitive?
You've got it right in your example; QUE opens a subordinate clause and as such it calls for another conjugated verb. Without the que there is no other clause and the verb is an infinitive as it is only acting as part of a prepositional phrase.