IE would need different handling than an open source app. The question comes down to, 'can we redistribute it?' If the license doesn't allow for unlicensed redistribution, it's a no go.
This is from there FAQ: http://www.tatanka.com.br/ies4linux/page/Legal_notices
IEs4Linux is free software, open source, covered by the GPL. But IE is proprietary, copyrighted, and you have to accept their license.
Note that to install any MS program included on IEs4Linux, you need a valid Windows license. IEs4Linux will not ask for it and you can run everything without any problem even if you don't have a license. But it is illegal and a I have nothing to do with this.
And you should accept the Adobe Flash EULA too.
More detailed answer
MS IE license says:
NOTE: IF YOU DO NOT HAVE A VALID EULA FOR ANY "OS PRODUCT" (MICROSOFT WINDOWS
95, MICROSOFT WINDOWS 98, MICROSOFT WINDOWS NT WORKSTATION 4.0, MICROSOFT
WINDOWS NT SERVER 4.0, MICROSOFT WINDOWS NT SERVER, ENTERPRISE EDITION 4.0 OR
MICROSOFT WINDOWS NT SERVER 4.0, TERMINAL SERVER EDITION), YOU ARE NOT
AUTHORIZED TO INSTALL, COPY, OR OTHERWISE USE THE OS COMPONENTS AND YOU HAVE
NO RIGHTS UNDER THIS SUPPLEMENTAL EULA.
That is: you must have a valid OS license (any Windows version greater than 95). But this EULA does not say that you have to install on Windows. So, you need a valid Windows license, but you can install on Linux.
IEs4Linux can't know if you own this Windows license or not. This software is provided only to people that own this license. If you use IEs4Linux without owning a Windows license, it may be illegal to use IEs4Linux in your country.
The way I read this, IEs4Linux is no different than PlayOnLinux. PlayOnLinux ask if you have a valid windows licence and it is in the repos so I don't see why IEs4Linux could not be there also.