But when someone makes an aveirah available, the aveirah is the stumbling block. The aveirah has its own temptations. The person who made it available is not being trusted at all. Thus the prohibition in this scenario is for simply making the aveirah available. The person who made the aveirah available did not entice the sinner or advise him to sin; he only made it accessible. However, the Torah prohibited making otherwise inaccessible aveiros accessible to others. This can only apply to aveiros that the Torah has prohibited, not to rabbinically- prohibited aveiros.
If someone were to make a bad deal available to another person without advising him in any way to take the deal, he would not be transgressing the prohibition of lifnei iver. Similarly, if one makes a rabbinic prohibition available, he will not have transgressed the prohibition of lifnei iver.
On the other hand, if someone advises another person to transgress a rabbinic prohibition, he will have violated the prohibition of lifnei iver because it is surely considered bad advice to violate a rabbinic transgression.
