כל הפטור מן הדבר ועשהו נקרא הדיוט - someone who is not required to do something and does it anyways is called a simpleton.This is brought down in the Beit Yosef (אורח חיים:32) where the issue of marking guidelines in order to write tefillin properly is mentioned.
(Talmud Yerushalmi Shabbath first perek halachah beth)
I think it remarks on a very basic difference between Torah of Israel an Torah of galut or exile. We know the Talmud mentions that someone who lives in Israel is like someone who has a God, whereas someone who lives outside of Israel is like a pagan who serves in sincerity. (עובד עבודה זרה בתמימות)
Perhaps outside of Israel, we have to hold to all the strictest interpretations of Torah, lest we lead a lifestyle of permisiveness to the point where we forget the Torah. When we live in Israel however, our life embodies Torah and we have to keep it exactly. It isn't enough just to do overmuch, we aren't trying simply to preserve the Torah, rather we have to be living the Torah at every moment. If we overexert in a particular of Torah observance that is beyond the boundary of what we are actually commanded, that same investment of energy could have been spent on observing the actual word of HaShem in some other matter.
In short, in Israel, we risk missing the forest because of the trees. Outside of Israel, where the forest isn't there to be seen, all we have are the trees.
Why does the Beit Yosef bring this up in the discussion of guidelines in writing tefillin? Perhaps because as we mentioned in a previous post, he has just brought up the explanation of the Rashbi that Tefillin are a representation of HaShem's own tefillin and we wear them whenever HaShem's own tefillin aren't revealed upon us. Similarly here, through tefillin, the Beit Yosef hints at the practice of Halachah outside of Israel, as opposed to the performance of the living Torah in Eretz Yisrael where HaShem's eyes focused from the beginning of the year until the end of it.
[This is not to say that the Torah performed outside of Eretz Yisrael isn't Torah, the spiritual level of Eretz Yisrael can be achieved anywhere (just as the spiritual level of Shabbath and Moshiah can be) however, living outside of Israel, one has to expend great effort and energy to reach the spiritual level of Eretz Yisrael. Living here in the land of our fathers, we start off in Eretz Yisrael.]