I wouldn’t call it a donation - users provide liquidity to the UNI/ETH pool for the same reasons that users provided liquidity to uniswap v1 and v2 for the past 2 years! They expect the money they earn from trading fees to be greater than the impermanent losses they suffer from price movement. Users are happily providing millions of $ in liquidity with no incentives, throwing money at them without needing to is a waste.
Also, while UNI/ETH liquidity is nice for token holders, it brings basically 0 value to regular platform users and traders. The currently incentivized pools (DAI/ETH, USDC/ETH, USDT/ETH, and WBTC/ETH) increase overall platform liquidity as bridge assets. Eg. if a trader wants to use uniswap while consolidating profits to BTC or stablecoins, the availability of a deep WBTC/ETH or USDC/ETH pool will help them access all of uniswap’s other liquidity with less slippage. Liquidity for UNI token is basically a bridge to nowhere, as it only helps those buying or selling UNI and not the platform more generally.