That is perfectly possible. The logic is as simple as "If you are on your mobile carrier network, 50Mb is the max, but no limit for wifi". If your laptop connected over wifi it has absolutely no knowledge of this arbitrary limit and will happily ignore it. The fact that it passes through your phone is of precisely zero interest to it and it won't know any better - neither will your phone have any clue what your laptop is doing.
The limit is I think one agreed to by carriers on the basis that if you are downloading on a phone you will be using your phone plan. If you are using the hotspot feature, then chances are you are paying for the privilege and using a tethering plan. Tethering plans are priced accordingly and you pay more simply for the fact that the expected usage is will be for larger files and bigger downloads or larger bandwidth activities etc. Typically a phone plan is slightly better value than a tether plan because you are simply able to do less on your phone to rack up a huge bandwidth total, especially if they stick the odd arbitrary limit here and there (no 50Mb+ iTunes downloads, no FaceTime etc) thus allowing them to keep phone plans artificially lower than they might otherwise be (as evidences by the price of the tether plans in comparison)