You ask two questions:
Is there a halachic reason to be opposed to Uri L'Tzedek and Tav haYosher? Why do some frum Jews oppose it?
I cannot answer the first. As to the second, the culture of the orthodox Jewish community is that it is resistant to change and that new trends tend to meet opposition at first. Here, for example, rabbis for centuries, and (more relevantly) even rabbis of last century, had seemingly not seen a need for such certification, so it can be viewed as not necessary. Moreover, it may be seen (even if inaccurately) as imposing rules (the certification's criteria) over and above halacha, which is often viewed as repugnant. Finally, the identities of the people who started it is no saving grace: had a gadol hador started the tav, a greater number of orthodox people may have acceded to his judgement as steeped in Tora.
I'm not passing judgement on the validity of these views (perhaps they are valid, perhaps not). You asked why some "frum" Jews oppose it, and this is my conjecture.
(Compare a similar though not identical answer about confirmation.)