Corona virus's effect on graduate admissions A great many top US physics PhD programs have canceled their open houses with no plan to reschedule.

Some are intended on running "virtual open houses." Other's are just sending links to information that could be found on the internet regarding their program.

Is the April 15th requirement of a decision regarding accepting grad school offers of admission still ethical?

Is it reasonable to propose that April 15th be moved a month or two later?

A STEM PhD could take anywhere from 4-7 years in the US. That is a non-trivial amount of your life.
You presumably did some research before deciding on where to apply to in the first place. That research is still valid. Any other contact with the institution(s) and faculty add to that basis of judgement. 
You seem to presume that there is one uniquely right answer on your choice, and that getting to that one answer requires attending an open house. Likely, almost any choice could turn out well, or turn out badly, all depending on factors totally divorced from the open house. 
Note that many people accept without having been to an open house. So, the decision can be made without going to one. 
You seem to presume that actions to reduce the spread of the virus are unethical, something that I would hotly contest.