The conditional you made is correct, but I don't think you're considering the entire stimulus.
We're originally given what you wrote above:
I & F --> U (i.e. Unobtrusiveness is necessary for an inviting and functional building.)
But don't ignore the second sentence. "Modern architects, plagued by egoism, have violated this precept." This is important. How might one violate the precept? By not doing what is necessary--by building a non-unobtrusive building. (Don't ya love LSAT double negatives?)
The followup sentence then tells us how they violate it--by letting their strong personalities take over their work.
Putting it all together, when these modern architects let their strong personalities take over their work, they violate the precept, which means to produce buildings which are not unobtrusive.