Sorry, but you and your lawyers are wrong. Many people use this incorrect terminology, even for real visas.
A visa is:
"an indication of the judgement that a foreigner wishing to enter Japan should be appropriate to enter and stay in Japan."
http://www.mofa.go.jp/j_info/visit/visa/01.html#b1
The visa itself is canceled upon entry into the country (emphasis is not mine, but MOFA's):
"Landing permission is stamped in a foreigner's passport by immigration officers at the airport or seaport where he or she goes through immigration procedures.
It is this landing permission, not a visa, that serves as the legal basis for the foreigner's stay in Japan. A visa is only a recommendation and does not automatically guarantee landing permission."
Japan has seven categories of visas. Note that Permanent Resident is not one of them.
http://www.mofa.go.jp/j_info/visit/visa/03.html#categories
Note here that Permanent Resident is clearly listed as "NO VISA GIVEN" (uppercase emphasis is MOFA's). "2. Statuses of residence without restrictions on activities in Japan"
Note also that PR is indefinite; that is, it does not have an expiration date like a visa does.
http://www.mofa.go.jp/j_info/visit/visa/03.html#categories
A more complete description of PR can be found here at the Immigration web site.
Immigration Services Agency of Japan
"
The status of permanent residence is granted when certain conditions have been satisfied by foreign nationals who reside in Japan under another status of residence and who have applied for change of status of residence to permanent residence or by those who have applied to acquire status of residence due to birth or renouncement of Japanese nationality.
If a foreign resident is authorized to stay in Japan on a permanent basis, he/she will stay in Japan with the status of residence "Permanent Resident." The status of residence "Permanent Resident" provides much more advantageous treatments than other statuses of residence because it does not limit the status holder's activities or period of stay..."
Lawyers and even some immigration officials incorrectly use the term visa a lot. It helps explain things a little more easily to the layman sometimes. One does not renew a visa, for example. One extends the status of residence. However, go to the immigration office, and you will both talk about renewing a visa, when clearly the forms (and some officials there) use other more appropriate terminology. In the case of PR, there is nothing to extend or renew, however.
As for PR being as easy to take away as a visa, I will beg to differ. You'll have to explain that more thoroughly, but I still think it is a weak argument.
Trust me on this. I've been answering visa-related questions for over 10 years and I have PR status. I've also come on a work visa and had a "spouse visa". This is not to flaunt anything; I just want you to realize that terminology is often misused, and I think lawyers are notorious for that in any type of law practice. It's their job.