The usual “official answer” from SketchUp (Trimble) employees is, that they are not allowed to give you legal advice regarding the product ↔ user agreements, and that if you need help to interpret them then you should consult an attorney.
There is a list of links to the various product agreements here:
Now, I do remember one of the product managers posting a clarification having to do with SketchUp product family license agreements. And that is in regard to your statement …
You are under a false assumption. If you read the Web Editions Terms of Service, you will find that it does not grant you (or any user) any specific “commercial usage rights”. Meaning that commercial use is not a right nor a product feature.
It is however a specific limitation for the Free edition only. (section 1.6.1)
No, they are not “paying for SketchUp”. They are paying for a subscription to use a specific product (service) in the family of the SketchUp division’s products.
There are separate agreements for each of these products types. Only the specific terms for each product type list the privileges and limitations for that product.
There is a special section that grants privilige to use other Trimble products in conjunction with your Shop subscription. These are listed in section 2. (“Use of other Trimble Services”).
Only those listed are the products that your subscription gives you the privilege of using.
So, the web editions and desktop editions are separate and are controlled by different terms.
The Desktop Tos also do not specifically grant “commercial usage rights”. The terms also have a specific clause that applies a limitation to the Free / Make editions with regard to commercial use.
The fees differ mostly due to the feature set, but also due to the cost of maintenance and support. (Ie, desktop applications generally cost more to maintain and require more support time and staff.)