The particular limits you are referring to often have to do with housing codes that permit year round living only in structures that meet local codes or HUD rated for permanent residence. On top of that, local codes may be "no permanent residence" for RV park vs Mobile Home Park zoning. Zoning commisions and city/county councils have their own ideas of what makes sense.
In other parts of the country, there are climate issues that limit the season, would not expect this for western Washington.
If the terms of use don't meet your needs, don't buy in. The "why" is irrelevant, as you are not likely to change it.