what version of SQL / SQL snapin are you using?

I have tested your script on a SQL2012 instance with the following installed

Name : SQLdmSnapin

PSVersion : 1.0

Description : This is a PowerShell snap-in for accessing SQL diagnostic

manager objects and actions.

Name : SqlServerCmdletSnapin100

PSVersion : 2.0

Description : This is a PowerShell snap-in that includes various SQL Server

cmdlets.

Name : SqlServerProviderSnapin100

PSVersion : 2.0

Description : SQL Server Provider

and it produces what you expect

ALTER TABLE [test].[table2] WITH CHECK ADD CONSTRAINT [FK_MyTable1_MyTable2_ID] FOREIGN KEY([four])

REFERENCES [test].[Table1] ([one])

ON DELETE CASCADE

ALTER TABLE [test].[table2] CHECK CONSTRAINT [FK_MyTable1_MyTable2_ID]