To be exact it is the weaker variant of Mox A with Mox C. If you buy Mox Start it is going to have 1GB of memory while Mox A in Shield has only 512M of memory. There are also not all components on A board as they would be on Mox A (to make it cheaper of course). And documentation states that it is “based on” not “same”.
Turris shield tries to simplify experience for end users and to provide cheaper alternative to get to Turris routers. The limitations are partly needed to make it cheaper and thus Turris Shiled can’t be combined with Turris Mox expansions (at least not officially). But primary reason is still the user experience. It is based on feedback from distributors. The limitation to only reForis interface, removal of possibility to install additional software and additional default settings are there to reduce complexity and to present cleaner and targeted product. The missing port forwarding in this sense is honestly just an missing feature in reForis. We plan to add it but there is no ETA at the moment so our official statement is: if you need it you should by Mox. Honestly Turris shield is marketed and intended for users who do not host stuff at home, for users not able to articulate “port forwarding” in message, to users who mostly just consume stuff. As I already stated, you probably want to buy Turris Mox rather than Turris Shield.