It's a low fat, low calorie eating plan...a few low fat products do have more sugar than you'd suspect without reading the label - but not many of those are free on slimming world.
Some flavoured yoghurts are the exception, but that's individual choice whether to eat those or plain yoghurt, which is also free.
The core of the plan is fruit, vegetables, lean meat, fish, eggs, pasta, rice, meat replacements, pulses and low fat dairy products, those are the freely allowed foods...out of those, the only thing that has added sugar is flavoured yoghurts of which only certain kinds are free and they only make up a small part of the dairy products.
You'd be hard pushed to be taking in high levels of added sugar while eating free foods, it's possible I suppose if you live off yoghurts, but that would be an odd choice with plenty of variety of free foods available.
Other low fat products that are usually marketed at people trying to lose weight, but are actually quite high in sugar and calories (mcvities go ahead biscuit things for example) have to be counted and are quite high in syns as well.
So actually it's only when something is low calorie, low fat and a little but high in sugar that it's "recomended". (Actually they don't really recomend stuff so much as let people make their own choices within the free foods)