That practice isn't very common, and I doubt that it has anything to do with reducing perceived risk based on volume. Most SP's charge a flat, all-inclusive rate and they either provide BBBJ (to most clients) or they don't.
There are a few Montreal agencies that have a policy of charging extra (usually $20 per hour) for each of DFK, DATY, MSOG, BBBJ and, in some cases, BBBJTCIM. This cafeteria style menu is not well-liked by MERBites, and the only reason these agencies can get away with it is because their base rates are so low to begin with (as low as $100 per hour for just CBJ and FS). However, none of the Montreal agencies whose rates are $160 and over have such a policy, AFAIK. Many just describe their ladies as GFE, PSE or YMMV, but still charge a flat rate.
I've seen many indies in several Canadian cities. The overwhelming majority provided BBBJ, and not one charged extra for that service. The only common extra charges are for Greek and sometimes facials and the like (neither of which I do anyway).