The assumption is the entry for the calculation field would be entered in human readable text so other users from the same country can create the formula themselves or look at another users example and modify that if they wish. Modification from a tax rule for the same country and year are likely to be minor. Slightly more for the same country with marginal rate updates and possibly quite significant between countries.
As to why would one user trust the opinion / answers of another user on a forum. Past behaviour and answer reliability is a start, just the same as assessing any other information source. I suppose the point being how a user customises their company file is a users responsibility. The value in crowd sourced customisation is if supported it makes it easier for a user to do, and thus makes Manager more valuable to users, thus increasing the potential revenue return to the software owner. The concept in various forms has been discussed several times including Community Supported Localisation .
A totally closed architecture is an alternative but that will either lack country / industry specific feature, or be much more onerous for the software company to maintain. The disadvantage of allowing customisation is it can increase set up complexity. Manager addresses this elsewhere by defaulting to the simplest implementation with the option to add capability if required. A good approach which should also work for customisable calculation fields.