Why must there be a limit? It takes a lot of money already to build the platform for a beacon.
The reason is that if people could buy infinite nether stars, that means that there would be a spam of beacon blocks all over the map, which would look horrible. I guess we will just have to wait until Optical finds a solution to this potential problem. Beacons do look really cool, but not everywhere.
You only need nine blocks in addition to the beacon block in order to activate it. You can get that by voting in 17 days, not to mention how easy it is to mine and/or trade with villagers. Multiply that by the number of players on the server and the number becomes significant.
Edit: Thank you, Goalie Guy. I did consider that the star price would be a factor, but Pensfan did not include that as a reason and thus I did not see a need to discuss it. Star price will of course be large because spawning and defeating a wither is not a simple task. However, we have a LOT of rich players on the server that may be willing to buy nether stars in large quantities and possibly sell them second hand at lower rates, (reason for doing so: unknown). Though I doubt that players who have been with the server for a long time would misuse the beacons by excessive placement, the players that they sell nether stars to may not be as responsible and therefore beacon spamming could concievably occur. Limiting the purchase quantity would therefore resolve the issue effectively.
